I sure wish some others would chime in with a response also if they can use outer points or no.
Here is an update - Bought the Sigma 50 to replace the 50L and it does the same thing with the outer points! The left side is a little worse as was the case the the 50L. But it consistently front focuses about a meter when over 15' away or so. And probably a foot front focus at 8' away or so. We also had another engagement shoot where I told my wife again to just use the center point so she did and every shot where she was over 15' away or so, the camera front focused like crazy! ... So I just shipped the 5D3 and the 50L to Canon on Wednesday. I sure hope they can resolve this because again, I put the Sigma 50 on the 6D and it works gloriously with the outer points! We have a few engagement shoots and 1 wedding scheduled during the time the camera will be at Canon so we will have to live with the 6D and 7D for a few shoots. Who knows, if they can't fix it, maybe 2 6D's would be better.
Sure wish some others with this combo would chime in to help out a bit on this!
I have the combination you're describing (5DIII/50L) and I can get consistent focus with the outer points (tried at f/1.2 and f/5.6). I'm using the left most f/2.8 point, the center point and the right most f/2.8 point and they give similar accuracy. I was shooting at objects inside at around 10 ft and outside at 20-30'. I'm using spot focus with f/2.8 points only. However, there is a big caveat. The entire focus point box must be in an area of high contrast to get consistent focus. If it is not, the focus can miss very badly (even with focus confirmation) especially with the outer points. The central points have additional phase detect baselines which increase their accuracy. I tried taking pictures of my daughter at around 10' where she was strongly backlit (without flash assist), and it missed badly because local contrast was poor. I then tried shooting a colorful bag on the couch lit by the backlight next to her (similar distance) and it nailed it consistently with center and outer points.
Perhaps a flash's focus assist will help...