Originally Posted by Sheldon Brown
With front derailers it gets more complicated, but it's a 2- vs 3 issue, the number of sprockets in back doesn't matter.
Sheldon-
Not to disagree but to ask for clarification, I haven't found this to be true. I tried to set up an old 8 speed front derailleur with a 9 speed chain and index shifting. It seemed as if the cage was too wide to move the chain efficiently. If I set it up to upshift quickly it wouldn't downshift consistently and vice versa. I had to set it up to downshift consistently and overshift a bit to force it to upshift. There may have been other incompatibilies, however, as I can't remember exactly which derailleur it was.
-s