From what I've read, ATX12VO really is 12V only; peripheral power connectors (sata/molex) will be connected to the motherboard, where the 5v and 3.3v (if needed) will be generated. 3.3V on sata power plugs was never reliably available and was rarely if ever used, and one of the pins was redfined to be a disable pin, so it's usually best for everyone if 3.3V isn't provided at all.

-12V for RS-232 is easily generated from +5V or similar sources; max232 chips have been available since the 80s