You can use the ground and power pins of a 3-pin motherboard header and ignore the RPM pin. Works fine. You will need a plug that fits the header, and that means a three-pin plug, even if you'll only use two of the three holes.
Alternately, you can power the fan using the power supply harness. The yellow wire is +12V and the black ones are ground. Connect the fan's red wire to the +12V and the fan's black wire to ground.
If this sounds like trouble, go to
www.teamawe.com and get some "tailed" Panaflos. He has them with both 3-pin motherboard plugs, and 4-pin power-supply plugs.