According to an article posted by photographer Jeff Cable on his blog, if you care about speed, you should avoid using a SD card on your Canon 5D mark III. Current Canon 5D Mark III (specs) has two card slot – one for CF & another for SD card. In his article, the CF slot which supports the newer very fast UDMA7 protocol and the standard SD card slot which does NOT support the high speed standard (called UHS – for Ultra High Speed). As a result you will end up with slower performance, even if you use hi speed SD card on 5D Mark III.
However, I am not a hi-speed shooter. So, it won’t affect me much. If you really need the speed… just use the CF card alone. Read the full article here