a antminer S1 need 360W at voltage 12 V
you can check Current minimmum for one antminer S1= 360W/12V = 30 A
check in PSU about current in body PSU.
if you need run 3 antminer S1,you need 3 x 30 = 90 A at 12 Volt
I checked in Power Description in
http://www.overstock.com/Electronics/Corsair-Professional-CMPSU-1200AX-ATX12V-EPS12V-Power-Supply-90/5243845/product.html?refccid=VW3Y7MKC3A6LUY2HTVQVRTOIFQ&searchidx=67#none12 V DC @ 100.40 A for run normally 3 antminer need 90A
So for this PSU, work for 3 antminer S1 for nomally running but not work in overclocked/may be will damage..
for overclocked need minimum 33.33A for a antminer S1.
if you need run overclocked 3 antminer S1 minimum Current is = 3 x 33.33 = 100A