This connector I've only ever seen on OEM PCs like Gateway and Dell. Most all retail motherboards do not use it. I guess you may even say it's the precursor to the 4-pin or 6-pin extra connector we're used to using these days.
Because the black wires should be ground, red 5V and orange maybe 12V? Similar to the extra plugs we use now. It's used for extra power.
But again, I don't see that on any retail motherboards, nor do regular retail power supplies have it. But I see it on OEMs a lot.
Yet another nonstandard approach the big boys use to screw the little man. Forcing you to buy a new power supply from them, because generic ones don't have their special connector.