How many Amps is the circuit? If it's 15A it would give you a max of 1800W but you should never run at 100% for a long time, 80% is recommended which on a 15A breaker would be ~1440W.
You should get a power meter (such as the popular Kill-a-watt) and check what each machine is using.
But I would not run 2 S5's and an SP20 on a 15A circuit, even on a 20A it's iffy (on 20A, 80% power is ~1920W).
Those S5's are probably drawing at least 1000W and then the SP20 is probably 700W-800W at the speed you mention, if not higher.
I am running 2 S5s and an SP20 off the same circuit in my garage and it keeps tripping the breaker.
What underclock settings should I use until I can get a dedicated circuit for my mining?
On my SP20 right now I'm at .66v (max .665) and 188W per loop.. Getting steady 1.23 TH/s
On my S5s right now I'm at 312.5M Frequency .. Getting steady 1.15 TH/s
What adjustments can I make to keep that circuit breaker from tripping w/out giving up too much hashing power?