Geforce GTX 780 Ti: 600 watt or greater power supply with a minimum of 42 amps on the +12 volt rail.
Radeon R9 280X: 600W System Power Supply Requirement
You may have already damaged your 3rd Radeon R9 280X by not giving it enough power.
Yeah, that is not a thing. The card will draw the necessary power it needs and if the PSU can not provide it then the PSU will shut down. A card can not be damaged by under-powering it.
Besides the card has:
- OVP (Over Voltage Protection)