It is perfectly OK to plug a 3-pin fan into a 4-pin fan connector on your motherboard. All fans are "keyed" so they plug into the correct pins. (fig.1) The first two wires on a fan are for positive and negative power.  The third wire is the tachometer and reports the fans RPM.  When a fourth wire is present, it allows fan speed to be controlled via a PWM signal. Plugging a 3 wire fan into a 4 pin plug on the motherboard will run the fan at full power and report its speed--unless your motherboard supports voltage regulation for fan speed control.