I've used the 50 f/1.8 II on my 7D for portraits, and a 35mm for full-body portraits.
So on a 5D3, the 50 would be for full-body portraits and you'd need an 85mm for head/shoulders (body if you can step back far enough).
Frankly, I'm not the biggest fan of any of the current EF-mount 50mm lenses.
- The 50/1.2L I borrowed from a friend last week, shot on his 5D2 and my 7D. Great bokeh, the f/1.2 really helped in low-light, but wide-open it's just a bit soft. Not bad soft, but it's way better stopped down (at which point, why are you paying triple for the extra half-stop?).
- The 50/1.4 i've never used, it's the sharpest of the 3 for (most of) the same apertures, but a lot of people complain about the build and weird usm. Hexagonal bokeh is better than the 1.8, but not as nice as the 1.2. This will be next updated (of these 3).
- The 50/1.8 II, well you get what you pay for in build quality, AF in really low-light on my 7D is pretty much non-existent (the 5D3 should do better). But the glass ain't bad, especially past f/2.8. If you're shooting stopped down past f/4 or so, i'd pick this one out of the 3. Pentagonal bokeh matters less unless your subject is really close.
If you're going to take full advantage of the 5D3's high-iso quality, stop down past f/4 where bang-on AF and razor-thin dof matters less, even the 50mm macro will do you good.
I've also got the 85/1.8 and 100/2.0. Same as Spokane, I'd be going either of these for portraits or more if you can step back a bit. IQ is great wide open on both, which can't be said for the 50s that have to be stopped down to get better.