front fan: input
rear fan: output
top fan: output (hot air rises and blowing it down again would be stupid).
Then you can choose, more input than output (high pressure, pushes air outside the case throug every hole), more output than input (low pressure, puls air in the case through every hole) or equal pressure (just rely on fans). I can't tell you wich is best, it realy depends on your case .
Careful with this...the top fan should NOT be a high CFM fan if it's right behind the power supply, otherwise you create a vacuum at the power supply's intake and it doesn't cool properly. This only applies to power supplies where there is a fan in the back and the front of it; for flow-through cooling. If you *must* use a high-cfm fan there, add a baffle (a tube will suffice) to drop the intake of the top fan below the intake of the power supply.