The 5D MK III takes the same size eyepiece as the 7D and 1D MK III / MK IV, so you should be able to use the extenders designed for the 7D, but I have not heard if they fit. I used one for my MK II, but the MK III is different.
I wear glasses and found that I don't need an eyepiece extender for the MKIII. I have the EP-EX15II on my MKII. It's great. Avoid Hoodman eyepieces at all costs. They're junk. They're cheaply made and don't work very well.
Yeah, I use an EP-EX15II for my Mk2 and love it. I really wish that there was one available for the Mk3, or at least something like it. The hoodman won't work for me because I mostly shoot with my camera inside a sound blimp.