I recommend the EVGA 1300 G2.  It has 6 PCI-e cables with a total of 8 PCI-e connectors (2 of the cables are doubles).  You can easily power 3 S3s at stock clocks with it.  If you under clock and under volt your S3 (not sure if the pencil mod trick works on these like it did the S1) and can get them to around 275 - 300W each, then that same PSU will drive 4 units.