One 8-pin power connector and cable of the PSU should be able to handle 2 connections to the GPU. however, running 3 cards with 2x8 pin connectors, you should check the power draw of the cards. could be too much for one 750w psu if you also connect the mb, cpu, hdd and risers to it. you can also check the splitters by adding them one by one and let it run for some time. another point: do you start your 2nd psu manually or do you have a psu adapter for it? add2psu adapter could be one cause, too