Do you shoot with a camera L plate? If yes, you should forget the third party grips as they are smaller than the Canon's BG. You won't be able to use the L plate on the 5D3 + third party grip.
do you know all third party grips... i doubt it.
i have a L plate and it fits perfectly.
Which third party grip are you using? The ones I have seen don't work with L plates.
5D3, 6D, 600D, RX100
16-35L, 24-70L II, 70-200L II, 100-400L, 50L, 85L II, 135L, 24TSE, 40, 100 macro, 18-55 II, 55-250 II, 600RT x 4
I come here to learn something new, not to learn how bad my gear is - I know that already ;-)!