EEK!
Three 40mm fans! 5500rpm @ 35 db (more like 50) for 5 CFM each!?!? I worked to get every 40 mm out of my case (the video card and north bridge), I wouldn't even think about ADDING them!
If you need more airflow into your case, and don't have room for a real intake fan, cut a hole out in the BOTTOM of the case to mount one. You just need to be sure the case is sitting on a solid surface (not carpet). You should easily be able to get an 8cm fan in, and maybe a 12! Mount near the front of the case, to blow cool air up at your hard drive or near the middle to help cool your video card. A 12cm fan can easily move 80 CFM, an 8cm fan 30cfm of air more quietly than those 3x4cm fans can move only 15CFM.
One more thing to consider; GENERALLY intake fans lower case temps, exhaust fans lower CPU temps. Which temp are you needing to lower? A faster/bigger fan in the rear might be all that is need if you have upgraded your heat sink and your CPU is still too hot.
Good luck,
Matt