Two intakes in the front, one exhaust on the top, one exhaust on the rear.
Fans on the heatsink should be oriented, so when you look at it, it draws air from the right and blows it out to the left. The entire case airflow should go Right (Intake, front of case) to Left (exhaust, back of case).
I've built in about 6 300s, and this will definitely be your best bet. The side fan is pretty useless unless you're overheating a GPU.
I spent hours last night trying every position fan speed possible in the case and found this the best setup.
2 x 120mm fans front running at half speed INTAKE
top and rear stock fans running on low setting EXHAUST
side running on low EXHAUST
COOLMASTER V6 running from front to back just over half speed
GPU running at 50%
just ordered 3 new 120mm fans as the 3 intakes are loud and don't have a good CFM