Ideally, you would need four fans. You should add two to your front pannel, to act as intakes. Then, you should add one to the roof of your case, and to the rear, these will function as out-flows. This will draw cool air in from the front of the case, pull it up to the mobo/GPU, and dissipate the warm air out from the case.
One thing to bare in mind NF-P12's are pressure optimised fans. These will channel air in a fairly concentrated pattern. For maximum airflow for the case, I would use air-flow optimised fans. Pressure optimised fans are more suitable for placement in restricted areas, such as behind your mobo or on a HDD cage.