You could do it with two PWs, a couple of hotshoe-to-household adapters, and some modified household powercables. Stobist.com’s Lighting 101 shows some ideas. You can fire two flashes with one PW – I’ve done it and it is a lot less than a third PW.
Basically, the off-camera PW is wired via household (two-prong) cables to both flashes. You’ll need a double-ended male cable.