The 5D3's SD interface is handicapped, testing shows the fastest you can get from it is ~ 19mb/sec, and that's with the fastest UHS-I class 10 cards. Slower cards will get you less than that. This is because the 5D3 Is not compatible with the latest write protocols, unlike its CF interface.
I have a Sandisk Extreme Pro 95MB/s SD card but the 5D Mark III handicaps it to a theoretical maximum of just 20MB/s. While obscene that in this day and age Canon can release a new camera with a slow SD slot, nonetheless I have had no issues recording 1080p 30fps or even 720p 60fps to the card, in All-I and IPB.
I bought the faster card for faster transfer speeds to my workstation, which is older than the 5D Mark III but for some reason HP decided not to handicap the speeds the way Canon decided to do
I also have a Lexar 1000x CF card and I have noticed no difference with it and with the SD card for video recording.
There is a huge difference though when shooting still in burst. I get 20-23 photos with AF on and IS before the buffer is full. I get 12-13 on the SD card. And the SD card takes a lot longer to clear the buffer once full. The Lexar clears the buffer within seconds.