9x 1W LED's parallel + series resistor calculation help needed
I would not use one resistor for all LED branches. Use one resistor for each LED branch.







simulate this circuit &ndash; Schematic created using CircuitLab

Uled=3V Iled=300mA  Uledbranch=9V  Iledbranch=300mAVcc=12V  

R= 12-9 / 3e-1 = 10 Ohms 
Pr= 12-9 * 3e-1 = 0.9W

Your 12V 2A switching power supply has required current output for driving 3x300mA branches. It might flicker or get hot.

You should also try experiment with connecting 4 LEDs in series and limit the current with resistor (it will be much cheaper and smaller).