So my 750W semi-modular power supply (Corsair CX750M) only came with 1 6 and 8 pin PCI-E power cable. What is the best way to add additional plugs for additional GPU cards? Are splitters safe? I will probably power no more than 3 cards before adding a second PSU.
First things first. If you get new cables, make sure they are designed for your specific PSU model. Even within the same manufacturer, different models can be wired differently and still fit perfectly. Particularly with SATA cables, but may also be an issue with PCIe cables. Many people have fried SSDs by using SATA cables interchangeably.A lot of people use SATA cables to power the risers (if you're using risers), particularly with lower powered cards (like nvidia 1060).
I wouldn't recommend putting more than 2 cards on the motherboard or more than two cards on a power cable unless you like pressing your luck or playing with fire (literally/figuratively).
Haven't heard of people using splitters, but if you're already powering two cards on one cable, I wouldn't push it.