Sounds like the card or riser has a issue. Try booting with just the 3rd card your adding. If it works add one more card at a time. Could be a faulty card, riser, or just hardware conflict.
could be pushing the upper limits of your psu? I'm going to be running a 1500 watt psu for 4 7950s and hardware just to be able to run them at like 68% with mobo going too.