I've got some fans on my case.... intake fan at bottom front... exhaust fan top center, and an exhaust fan on the back... I have two on the side of the case, near the agp/pci slots, and I'm not sure whether they should be intake or exhaust.
If you have a thermometer with a remote sensor, try the fans as intake and record your temp. Then reverse them and see what result you get. With three exhaust fans already, I suspect you might have better luck on intake, but the results will speak for themselves. Another point to consider. With too many exhaust fans, they start to pull air in around and through your CD and drives, causing a dust buildup problem.
Those two fans at the side of the case should blow the cold air in..check your temps then..there could be a slight problem ..but its not always the case with those cpu coolers that thave its fan sucking the air from the cpu and then those two side fans could blow that air back and neutralize the effect..but like i said it happens rarely..very rarely so go for intake first and check the temps..
I just mounted a 120mm Sunon fan right above the agp card on the side of my case and experimented with temps. It definitely was cooler blowing air in than sucking it out. I can't believe how much of a difference it made. I'm at 1080mhz now (108x10) on my 650 Duron and hoping to hit 1100 this weekend. I could only hit 1050 before and temps were over 50oC. Now they hit 44 max.