Agree with QuintLeo. And just to add...
I have used some cheap-ish "pure Molex" risers which were making 3.3V from that 5V rail coming from "the Molex". 3.3V power line should not see much load if any. I measured it on a couple different GPUs and it was well under 1A, which is less than 0.3W.
Those cheap step-down LDO voltage regulators they put on risers still have decent efficiency at low currents. Say, 80-90% if not more. Do the math and get 0.3W - 85% = 45 mW of power loss on the riser!
dude, you need to do some reading on how PCIe works.
power comes from 12v ONLY!
3.3v is only used for signal sending! you are not pulling any large amount of current from 5v or 3.3v lines.