post #11 of 11
The +12V rail supports a bunch of important things in your computer, notably the CPU, the GPU, and the hard drive and optical drive motors. In the end your PSU only has one source of 12V current, not three, so it's not really split. It's just that there are three copper traces that are monitored individually, with OCPs calibrated at 19A each. It's pretty much useless to worry about it too much since it makes very little difference in 99% of situations.
+3.3V and +5V power different things in your system and you shouldn't worry about them. What matters is that they're there.
I suggest you read these two articles to answer your questions: