My advice is to get a camera. If it is a play, where the actors do have mics on them it will be much more easier to get a long XLR cable and stick it in your camera. You dont have to worry about the sound. (in most scenarios). Sound is (in my opinion) important in a play. I dont think a standard 5D mkIII will give you the best sound.
So to sum up. Get a camera.
“You never want to get to get a call from the editing room”
Art Adams