The EVGA850 came with one Perif Cable which is 6 Pin to PSU x Molex with 3 molex on the cable.
Can I put 3 USB risers to that one cable or should I go buy 2 more 6 Pin x Molex cables for the other 2?
Don't put the 3 USB risers on one cable molex, this is a bad idea, been there and melted the wires, best is to have 1 is to 1 rule, 1 molex for 1 riser.
here is the guide, kudos to blog owner
http://www.gpuminingresources.com/p/psu-cables.html