how to use diode
gameelgamal,
Barrybear is correct. to use a Zener diode in a simple shunt regulator configuration, the series resistor would have to dissipate (12-6)*3 = 18 Watts. Furthermore, if you remove the 3A motor load, the Zener would have to also have to dissipate 18 Watts. This simplified analysis assumes that the Zener current under the 3 A motor load is negligible. The only practical way to provide the regtulation is to use a switching buck regulator. It would waste very little power and greatly extend the battery life.
Regards,
Kral