With the battery powering it, I want to have it power 2 other boards that both need a 5v connection and also power one board that is 3v.
It all depends on how much current the three boards require. The 3.3vdc voltage pin is very limited on how much current it can supply, around 50ma I belive. The 5volt pin current capacity of the Arduino is better at around 700-800ma. You could always connect an external 3.3vdc regulator if you require more current for that voltage. So research the current requirements for those external boards and get back to us.
Lefty