my bad, i forgot the voltage drop of zener diode.
If you really need 3amp, you can look into the datasheet of your voltage regulator to see how to expand their current capabilities.
it didn't work
so i cannot use npn transistor?
Let's go back a few stepsQuestion - Why do you think you need a 3A regulated supply ? What is your actual load ? Just because the power supply can provide 3A+ doesn't mean your regulator has to be rated at that figure. If 6 volts meets you back-up needs than "losing" 12.5 across a linear regulator is a shear waste of power (12.5 x 3 = 37.5 watts). If you really need 6 or 12 volts (you seem to be a bit mixed on voltages) then I'd suggest a switched mode regulator that offers over 90% efficiency with very little heat.