500W is too small for an S3, I'm surprised you haven't melted it down yet. A good rule of thumb is to not exceed 80% of the rated power of the psu.
It didn't even seem to get warm. Wait a minute, I was only running one s3 on that 500 W (EVGA 100-W1-0500-KR) power supply. God, I'm such an idiot! Better edit that post :-/
EDIT: The only reason I posted here is that I don't have power for those s3's and folks here sounded like they did. I actually had one of those "used" 1 kW HP server power server supply which ran both. The 500W just ran one s3+. I have an extra 750 W EVGA 750 G2 - do you know if that'll run both? Prolly not...
Anyway, if this is the wrong thread, maybe sysop will move it to the right place?
Thanks, and apologies for the confusion.