Yeah, three flashes at 1/4 power isn't "almost" full power, it's 3/4 power. If those YN560s can do 1/3EV increments between power settings (which the 430EX-II can: it'll go 1/4, 1/4 +0.3, 1/4 +0.7, 1/2, etc.), then doing 1/4 +0.3 on all flashes will give you a theoretical total of full power.
If not, then your best bet is use them at 1/4 power and expose a bit to the right, or use them all at 1/2 power and expose a bit to the left. You could of course also experiment a bit with them, putting the two side flashes at 1/4 and the top one at 1/2. That would theoretically give you a total full power, but may result in slightly uneven output.
7D - XTi - Tamron 17-50mm f/2.8 - Canon 50mm f/1.8II - SMC Takumar 50mm f/1.4 - Canon 70-200mm f/2.8L - Lensbaby 2.0 - Canon 430EX-II - CBS flash trigger