If anyone is interested, Markins got their camera plates for the 5D3 in last Thursday. I received mine today and I'm very pleased.
There are many good plates available but I prefer Markins plates because they are lightweight and low profile yet they are bulletproof. Like heavier, larger competitors, there is a place to attach a hand strap or screw in a 1/4-20 stud to the bottom of it. Unlike many competitors, the attachment point is square so the plate fits into the clamp in either direction.
Compared to the Markins plate for the 5D2, the new 5D3 plate is somewhat larger, though not thicker. And, unlike the RRS plate for the 5D3, the inside edges of the new Markins plate are curved to perfectly match the contours of the 5D3 body. This was a big deal for me. I didn't care for the RRS approach.