3dB, though, is double the noise level.
It's not just about the number of fans, though. The primary consideration is volume of flow. You can run your case pressure as positive, neutral, or negative, as long as you have adequate exhaust to prevent heat buildup. If exhaust temperatures rise just a few degrees C, then you doing very well. If your exhaust temperatures are hot to the touch, then you should increase exhaust flow through rpm, or additional exhaust area.
Usually, if the number if intake fans is the equal of exhaust fans, and you have the fan orientation correct, then you should be fine. I have been using a front panel fan controller for years now and usually run the intake fans at a higher rpm than the exhaust fans, but I also have Scythe fans on the hard drive mounts to make sure there is adequate flow there, too.