3.3V is pretty much only used for DRAM. That includes powering the RAM slots on the mobo and the RAM on your video card. SATA HDDs are also designed to use 3.3V on their logic boards but that has pretty much been abandoned.
5V powers most circuitry. That would include the logic boards on HDDs and optical drives and various chips on the mobo. USB is also powered from the 5V rail.
The 12V rail is for anything power hungry and all motors. That would be the CPU, the core on your video card, fans and HDD/optical motors. It may also power some various bits on the mobo but that will vary with manufacturer and design.
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