I seem to recall reading somewhere that it was unwise to do that.
Do you remember what the reasons were for that? Like I said above....I am very new to all of this.
The computer power supplies are constructed supposing that you will use a "+ pin" and a "gnd" to feed some equipments, boards, etc.
When you use +12 and +3.3v to create a "8.7volts power supply", the computer will supply currents different than what was supposed to work.
Another problem you can have, is when you make a mistake, connecting a +3.3v as a "relative gnd" and in some point of the circuit you can forget you did this, and connect the real gnd to the same point, creating a short-circuit on +3.3 power supply.
As you said you're a newbie, I suggest you don't do this.