I agree that you should use the money for the third fixture to get a pressurized system instead. I was pretty new with the pressurized system, but I got it up and it's running, so it can't be too hard if i can do it. LOL.
You can buy a complete system or buy parts and put it together. Here's a good link of the cost and how to obtain the system by pieces:
http://www.plantedtank.net/articles/Pressurized-CO2/19/
Here's a link to a video on how to set it up, it's a really good visual:
http://www.youtube.com/watch?v=qdOFCatSPKo