How many fans are you talking about? If you're just going to use the RPM adapter, I don't think it makes a difference how many 3 pin headers you use the RPM adapter on since it's only getting RPM readings. But if you're talking about connecting the fan power directly to the header, I think smaller fans like the chipset one isn't a problem, but high output 60mm and normal 80mm fans might put a good draw on the motherboard. The PSU fan isn't a problem since it's just reading the RPM, not drawing power...it gets it's power from within the power supply.
Cricket