The RM850 is a good unit (unlike its little brother the RM650), but it uses some crap quality Ltec capacitors. The Super Flower-based EVGA units are the top picks here.
The RM650 is missing two wires in the ATX spec, and the RM850 has better build quality. Both pale in comparison to the EVGA G2 series. You can check out the jonnyguru reviews to learn more.
Three graphics cards, sure. 850 should cover two no problem.
Two 290Xs in Uber mode and an i7-4960X @ 4.2GHz consumes ~730W.
http://anandtech.com/bench/GPU14/885