For stills, look for either a SD card that states it is Class 10 or a CF card that is UDMA 7 because either will easily keep up with your 5D III at its maximum frame rate.
IF you are planning to also shoot significant amounts of video, then the CF card is slightly faster but the SD card interface is more mechanically robust and I prefer using it. For still photos, it will perform as well as a CF card. Buy it from one of the well known photo suppliers like B&H or Adorama or use a trusted local store to avoid a counterfeit. I prefer SanDisk brand but I have also used a lot of Lexar cards without problem.
You have made a good lens choice for sports. For a basic sports setup, set your 5D III for:
1. AI Servo focus which is your camera's best setting for tracking action sports.
2. For your focus point, choose AF point expansion with the center point plus the four surrounding diagonals, this is the best compromise between ease of following the action and best AF performance. Choosing expansion to 9 points makes it a little easier for you to follow the action but will slow the AF performance a bit, choosing only a single point provides the best AF performance but puts more of the work on you to keep that single point exactly where it belongs.
3. Set the mode selector to manual. Set aperture to f2.8. Set shutter speed at 1/500 for very dark venues and 1/640 or 1/800 for typical night time school stadium lighting. In sunlight, you can close aperture to f4 and also increase shutter speed.
4. Set ISO to auto, the camera will choose the proper ISO for a standard exposure.
5. Set light metering to partial metering, this provides a good sports metering selection that provides proper exposure for the athlete when the background is either very dark or very bright which are both common in sports.
6. If you haven't been shooting RAW instead of JPG, you will want to shoot RAW for sports which allows you the best post shot correction for exposure, noise, and white balance. Canon's DPP software that is free with the camera provides excellent RAW conversion for Canon cameras and the basic editing tools you will need.
7. Set drive mode to high speed but concentrate on timing shots rather than long bursts.
8. Night lighting at fields varies a lot in color temperature so don't be surprised if you end up with some very odd color tints, this is another major reason to shoot in RAW so that white balance/color temperature can be corrected in post. One feature the 5D Mark III lacks that later models have is a white priorty setting for auto white balance which is very useful with sports but correcting white balance in DPP is fast and easy. The practice of using a neutral card to establish white balance doesn't work for typical school sports facilities because the color temperature varies across the field and as the lights cycle so plan on spending a little time with correction.
9. Location is the most important thing in shooting sports. You want to be as close as possible because the more cropping that you are forced to do, the more noise becomes an issue. You want to be in a place where you feel safe and are not blocked by other fans, players, etc. You probably won't be able to get field access and if you do, be VERY careful. I have dodged a lot of players and balls this year like every year. You MAY be able to attend a practice to get some good photos in a more relaxed setting and when the light is better, ask the coach because many of them are happy to accomodate you-especially if you shoot more than just your own children and share those with the coach.
I am not sure what your shooting experience with action is in the past so please don't feel insulted if these suggestions are aimed below your level. But with sports shooting, the card is the easiest part and there really isn't a need for anything special. Reliability of the card is much more critical than absolute write speed which is why you want a major brand from a trusted retailer. If you start shooting video, then a fast CF card has a slight advantage over the SD card but the CF connector pins are easier to damage which is something to consider if you are taking cards in and out of the camera to transfer photos.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
