Is there a good reason to own a combination of these lens, or are they too similar to justify high prices?
I have the Sigma 85mm and the 135L -- the pair can be had for less than $2000.
Since you're wondering specifically about AF performance -- the 135L is a speed demon. The focus limiter is a nice touch for this (btw it also focuses at about 1m)
I don't believe the three lenses are all that similar. The zoom is a zoom with IS (it's also slower, heavier, and more conspicuous). The 85L is short enough to be usable indoors but doesn't focus very quickly (for example it's not a sports lens).
The 135L autofocuses very sharp but is a very long lens for using indoors. I usually use it as an outdoor portrait lens -- I mount the 135L if I'm outdoors and the Sigma if I'm inside.