Can a 3-pin case fan be used on a 4-pin PWM port?

A 3-pin case fan can easily be connected to a 4-pin connector on the motherboard. The fourth pin is simply left blank in this configuration. In most cases, it will then run unregulated at a fixed speed. Some newer boards offer the possibility to adjust the fan speed by voltage. Look for the option "Fan Control Mode" and set it from "PWM" to "Voltage" or "Analog". See the manual of your mainboard for detailed information. It is also possible to install a 4-pin fan on a 3-pin connector. Here the fan runs at full speed, too, provided the type of control has not been modified.