"I have the kit lenses, 18-50mm and 75-300mm."
This is actually all you need right here all ready! Instead of buying a converter for the simply awful Vivatar get a cheap set of exrension tubes for your current lenses. And shoot away...............
Hold on a second...
Vivitar sold some excellent macro lenses, back in the day, as well as some that were only so-so.
Vivitar actually has never made lenses themselves... they outsourced all the manufacture to other companies. Sometimes the manufacturer was truly top-notch... Other times, not so much. For example, up to about 1990, lenses with a #22xxxxxxx serial number were made by Kiron and are pretty much on par with the best Nikkors of the era. This is just one example, there are other excellent Vivitar lenses made by other manufacturers.
Without more info about the lens you found, it's sort of hard to advise your best course of action.
First of all, are we talking about a complete lens, or a macro adapter that screws into the front of another lens? If it's an adapter, it's probably not worth using. If it's an actual lens, read on...
Some Vivitar lenses were interchangeable mount designs. Early ones used a T-mount that simply screws on. Later ones used a more complex T4 or TX mount. You can get T-mount to EOS and T4/TX to EOS interchangeable mounts for these lenses, often made in China.
Other lenses were built with non-interchangeable mounts, but in a bayonet mount that's easily adaptable for use on Canon EF mount cameras. If your lens is one of the non-interchangeable mount type, the first thing you need to do is identify what mount is on it (actually you can do this even if it's an interchangeable mount, too). Look for some indication near the base of the lens.... such as "CA" for Canon FL/FD, "P/K" for Pentax bayonet mount, "KO" for Konica bayonet mount, "OM" for Olympus OM, etc.
Once you identify what mount is on the lens, some can be easily adapted.... others not so much. There are eleven common "vintage" mounts that are easily adapted including Pentax P/K and screwmount, Oly OM bayonet, Nikon F mount, etc.
It's actually a lot easier to tell you the five bayonet mounts that are not easily adapted for use on modern EOS cameras. Those are: Canon FL/FD, Fujica-X, Konica AR or F, Minolta MD and Miranda. Most other mounts used for 35mm cameras can be adapted for use on modern Canon cameras. You can find Chinese-made adapters many different places.
Even a lens with interchangeable mount (T, T4, or TX), so long as it's in one of the easily adapted mounts, can simply be adapted if you can't find an EOS interchangeable mount. With the interchangeable mounts, even if it has an unadaptable oddball on it (such as the Fujica-X), you could replace that with, say, a Nikon F mount and then use a Nikon F to Canon EOS adapter.
More info about adapting lenses for use on modern Canon cameras can be found here at Bob Atkin's website.
Once it's been adapted for use on your T3, it isn't hard to use these old manual focus, manual aperture macro lenses. They are slower to work with than a moden AF, electronic aperture lens made specifically for Canon. But image quality can be just fine. With the adapted lens you will be limited to use either M or Av exposure mode on your camera... can't use Tv, P or any of the highly automated modes.
Here's a Tamron SP 90/2.5 macro lens I bought at my local second-hand store, on one of my 7Ds...
This is an interchangeable mount lens (this Tamron uses a mount system called "Adaptall" and "Adaptall2", different from the interchangeable mount systems Vivitar used). The lens had a Nikon F mount on it when I bought it. I got an EOS Adaptall mount from China and swapped it out for use on my Canon cameras (actually it's now been swapped back to Nikon F and is on a vintage Nikkorex I've been shooting with).
Here are a couple images made with the Tamron 90mm (with 20mm extension tube, on 7D)....
Not bad for a $20 lens (almost "like new", with 1:1 adapter, caps, hood, Nikon Adaptall mount)... and another $40 for the Chinese Adaptall-EOS adapter. Here's the lens with the EOS Adaptall on it, and the Nikon Adaptall alongside...
Still, it wouldn't be a bad idea to also get a set of macro extension tubes. They can be very handy to have and some are quite affordable. The Canon 12mm and 25mm tubes, which sell individually, and the Kenko set of three (12, 20 & 36mm) are the best quality, but also the most expensive. A bit more plasticky, but still workable are the Opteka (12, 20 & 36mm) and Zeikos (13, 21, 31mm) sets... both brands sell for about $75 or less for the set.
Notes: the Zeikos sell under a bunch of different brand names. You'll also find them labelled Vivitar, Bower, Dot Line, ProOptic, Jessops, Vello and more. Shop around for the best price. Also, there is an even cheaper version of these that uses plastic bayonet mounts, instead of metal. I'd recommend spending the extra for the metal ones.
For any of these, in order to use them with your EF-S kit lens, you'll have to be sure they are compatible. The current Canon mark "II" and Kenko "CA/AFs" are. I am pretty sure the recently introduced Opteka are, too. The Zeikos mostly seem to be now, too. Just be sure, if you opt for those.
Macro extension tubes can be very handy to have... I've always got a set with me, even when I'm carrying a macro lens. I've used them with non-macro lenses from 20mm to 500mm. WIth lenses up to about 100mm, they can get you to fairly high magnifications, at least close to macro range. With longer telephotos, they can improve the lens' closest focusing ability. With macro lenses, they can boost magnification beyond the lens' 1:2 or 1:1 capability.
Hope this helps.
If you could provide more info about the Vivitar lens, perhaps even a photo or two of it, we may be able to offer more advice.
***********
Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM
" I looked at the lens at the base, i could only find "c/fd" and "no. 92Xxxxxx"
"c/fd" would indicate Canon FD mount, which normally is not practical to adapt for use on modern cameras... except in this case it's a macro lens we're talking about.
So long as you only want to use it as a macro lens, never need to focus it to infinity, you can get an adapter like this one on eBay. This type of adapter has no optical elements, so will not degrade the image in any way, but it won't be possible to focus the lens all the way to infinity. This adapter also has a "chip" that will allow Focus Confirmation to work. I recommend this type which cost a few dollars more, as it can be helpful to have Focus Confirmation since modern cameras aren't set up with some of the manual focus assist features of vintage cameras. Only you can say if it's worth $17 US to have a manual focus macro lens.
To use the lens, first put your 18-55 or other AF lens on the camera and set One Shot focus mode (this is so Focus Confirmation will work). You also might want to select the center AF point only. I think on most modern Canon cameras you need to have an AF capable lens in stalled on the camera to make those settings. Now replace the AF lens with the adapted macro lens.
Next set the camera's exposure mode to either M or Av. These are the only two modes that will work correctly with an adapted lens, where you have to use the aperture manually. You still will be metering through the lens with the camera's built in metering system. If using M, you'll need to set up the ISO, shutter speed and aperture you'll be using to take the shot (see below). If using Av, you'll get auto exposure.... You set the ISO and aperture and the camera will select whatever shutter speed is needed... just watch to be sure the shutter speed isn't getting too slow. If it does, increase your ISO. (Above sample shots were done at ISO 400, f11 and 1/400 shutter speed... Oh and by they way, they are "straight from camera" RAW conversions, through Lightroom, without any exceptional post-processing. Pretty "awful" third party lens, huh? 
)
I am not a big fan of Auto ISO and don't recommend using it. One reason is because there's no way to use Exposure Compensation with it. Of course, if you are using M, there is not Exposure Compensation anyway. But if you use Av exposure mode, in particular, I'd steer clear of Auto ISO.
Next set the lens' aperture to it's largest (probably f2.8, f3.2, f3.5 or f4). This will give you the brightest possible viewfinder to focus manually, but it also causes depth of field to be very shallow. This is helpful while you are manually focusing. But before you take the shot you will want or need to manually stop the lens down to whatever aperture you have chosen to use in advance. To increase depth of field, I'd recommend at least f5.6 and maybe something in the f8 or f11 range. This will darken your viewfinger too, but usually leaves enough light to compose and check that focus is still okay. Experiment with different apertures. The higher the magnification, the shallower depth of field will be and the smaller aperture you may wish to use. But you want to stay clear of super small apertures f16 and beyond, which will cause loss of image quality due to diffraction.
Rather than turning the focus ring to focus the lens, try moving closer to or farther from the subject.... this is often an easier way to focus macro (even with modern AF lenses). When you see focus where you want it and/or hear the Focus Confirmation "beep", take your shot. When I'm shooting handheld with a manual focus macro lens, I'll often set a high frame rate and take a short burst of 3 to 5 shots, to better insure one or two will be focused right where I want. If using a tripod, this might not be necessary.
Note: at f8 or f11, especially if light conditions are less than optimarl, Focus Confirmation may stop working.
Have fun!
EDIT: "92xxxxxxx" was not one of the manufacturer's codes Vivitar used... so one possibility is that the lens was made before they started using that system. However, I did find that they used a single digit code on a few lenses, and "9" was the code for Cosina manufactured lenses. So that's another possibility. Either way, it could be a pretty good lens and might be worth a try.
***********
Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM
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