I need a cable that will allow my power supply to power a couple of my CoolerMaster 120mm case fans. Since I don't feel like spending $5 a piece or waiting for shipping, I thought I might try to make them my self from the cables from an old power supply I have. I already have a male to 2 female molex adapter, so I need to make 2 male molex to male 3 pin fan cables. On the cables I make, I think I would only need to wire the 5 volt red and black wires from the molex to the fan connector, but I'm not sure. Do I need the 12v yellow wire for anything?

No.

12v yellow to 12v black(or red). Bigger fans need more voltage.

This picture should help you:

http://www.amazon.com/Fan-Molex-Pin-MB/dp/B000BSJGL0

Why not just plug it into a molex connector? Most of those fans have a coupler so you can still plug into another device.....

They only have a small 3 pin female connector.

5 volt may be too low to start the 12 volt fan.

Yep, that's the problem. I just ordered adapters from Newegg so I don't have to fool with it.