I will recommend only 2 Ant S1 in that 1200W PSU.
Loading 3 Ant S1 even at stock clock speeds are an overkilling.
1300W Peak in that PSU is not for 24hours. So just calculate like this.
1200*85%=1020W.
Each Ant S1 consumes 360W So. for 3 Ant S1 its 360*3 = 1080W.
Inshort you run 2 Ant S1 or under clock one or all to make it run 3 Ants.