I'm pretty new to mining (besides a 2 week stint in 2010/11
so take my advice with a grain of salt.
You could get cheaper/more performance from buying new video cards. However they will take a lot more power (roughly x5 this amount).
So basically it boils down to whether or not you live in a place with cheaper power, I think. This math you have to look at is that hashing rate versus the hashing rate of 3 video cards versus the price of power in your area. Totally off the top my head as a ball park figure I think one PC with video cards supplying 1.5 GH/s would be somewhere around a 800W- 1000W total system drain but this is a total bar napkin-based estimate.