Yes, you can plug the 3 pin in the 4 pin connector (notes: the pin left is which controls the RPM of the FANS, so maybe you can't able to control it)l
What do you mean the pin left? do you mean like if i plug 3 pin fans to 4 pin headers there will be a pin missing and with that pin missing i wont be able to control the fan speed? is that what you mean? and if i had the 4 pin fans with the 4 pin header i would be able to control the speed?