i'm assuming the separate 12v lines are for high draw components like GPUs. So the wiring doesn't get burned up by high current draw, they're separated so you can have 1 for the motherboard, and two separate ones for something like a 1080 or the new vega that require separate 12v rails. Some of those beasts can hit like 400watts (35amps) peak, which is probably too much for a single rail.