Canon doesn't support a lens extender for this lens model so if one is available it would have to come from a third party. Because even a 1.4X would put your lens at F8 once you are barely past the widest angle, your 3ti won't autofocus with an extender.
Even the best quality primes suffer a loss of sharpness and quality with a 1.4X but because they are so sharp to start the dropoff is somewhat acceptable. I have used a 1.4X with my EF 300 and 400 f2.8 lens at times but even with those there is noticeable degradation.
Depending upon what you are shooting, the combination of any slow lens and the further loss in stop(s) due to converters may put you into a light challenged situation at times when you need a lot of shutter speed to freeze subject motion. There are occasions where I go with the shorter focal length of my EF 400 f2.8 and use tighter cropping rather than give up the two stops of light from using my EF 800 f5.6.
An often overlooked Canon prime that has a nice price to performance ratio is the EF 400 f5.6. It is very manageable for a 400mm, provides nice sharp high contrast photos, and has very fast focus acquisition. I bought one with my first Canon 1 series digital back in 2005 and I have kept it because it is a nice performer and more comfortable for taking on hikes compared to my 400 f2.8 which is much larger and heavier. The EF 400 is one of the L series lenses and is well built and I wouldn't hesitate to buy a well cared for used version.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
