you might never see an aggro deck with more than 2 colors (and doing well).
I think if you're trying to build an aggro deck with more than 2 colors then you're doing it wrong anyway most of the time. Aggro decks should be aiming for consistency as much as possible; our most successful aggro decks are usually as close to mono-colored as possible with a small splash for the best support cards in your second color.
This is probably true if you're playing in an environment without dual lands that ETB untapped. It's certainly possible, though uncommon, to draft a three-color aggro deck in an environment with them. It usually looks like a two-color deck splashing for a third and has to have a lot of fixing lands. If the fixing doesn't come together, you're almost definitely better off going with two colors instead. Supporting the occasional three-color aggro deck is one of the benefits I've noticed of adding the painlands to my cube, along with just increasing the overall consistency of all decks and making aggro marginally better. You can certainly get some improvement to aggro decks just by trimming some of the CC cards, but your aggro decks are still going to be better and more consistent with painlands than without.
CubeTutor: www.cubetutor.com/cubeblog/72
Thread: http://forums.mtgsalvation.com/showthread.php?t=512410