The Canon 5D Mark III has two card slots— one for Compact Flash and one for SDHC / SDXC cards.
In addition, I tested a SanDisk Extreme Pro 64GB Compact Flash card and an (older) SanDisk Extreme III 16GB card.
Speedy and not so speedy
Shooting RAW, the camera popped off as many frames as it could until it stuttered. I then timed how long it took to clear the camera buffer completely. File sizes were about 27MB for each frame.
SDXC card speed in the Canon 5D Mark III was sluggish, even using the fast Lexar 400X card, which performed extremely well in the Fuji X-Pro1.
Lexar 64GB 400X SDXC: 13 frames, 20 seconds to clear SanDisk 32GB Extreme Pro 45MB/sec: 12 frames, 20 seconds to clear
The 5DM3 could fire off only 13 frames before stuttering, and then took 20 seconds to clear its buffer. In the 5DM3, using SDXC / SDHC cards appears to be a bad choice for any sports or action shooter seeking high frame rates and burst capability.
Compact flash performance far faster than SDXC / SDHC.
As can be seen, the Lexar Professional 1000X card was by far the best performer.
Lexar Pro 1000X 32GB: 26 frames, 4 seconds to clear
SanDisk Extreme Pro 64GB 90MB/sec: 20 frames, 7 seconds to clear
SanDisk Extreme III: 12 frames, 21 seconds to clear
Get Compact Flash and SDHC / SDXC cards on the Other Gear page.