Good question !
I had issues before as i had a OZC 500W and just 1 390 and a 7950 which was too much.
Upgraded to a OZC 750W and no issues, even with 2x 390. I added the 7950 using the seperate 500W and worked well
Added the 3rd 390 and then it was an issue again so i got an used 1400W Antec psu and no worries.

All well and after days it all started falling apart with data errors and lock ups. Removed cards, changed risers.
Let 2 cards out, went back to 750 and 500W config ... Then back to just 1 390 and 750W and no joy for long.

Hence why i am lost and it looks like i need to go back to the start.
Strange that i was mining for 2 weeks with 3x390 and no issues at all and now nothing seems stable.