Does the connector on the fan look something like
this? Might be 3 holes instead of 4 but should look similar, assuming its just a typical case fan and not one of those fancier ones
If so you can plug it into a
fan header on the motherboard. It looks like 3 or 4 metal pins sticking up but make sure it says something like "System fan", "SYS FAN" or "Case fan", it should indicate such near the connector on the motherboard.
If the pins on the motherboard are 4 pins you can still plug a 3 pin fan into them, one of the pins will just be exposed which is fine.