There's no exact answer to your question, but the general consensus is that you want cool air to enter at the bottom, and hot air(which rises) to exhaust at the top. I'm assuming you have an ATX power supply which should have been set up to exhaust hot air out through the back. If the case permits, mount an additional fan up top exhausting air outward.
Whenever you have an odd number of total fans(including power supply) have the extra fan exhaust air outward(prefferably at the top). Your case will almost always have weep holes where air will leak in...if it is pulled inside. This seems to be better than having too many fans blowing in, which will over pressurize the inside of the case.