I have a project that I want to convert the voltage 9vdc to 3vdc. Can anybody help me which circuit diagram can I refer and use to drop the voltage.

 

Check out the large number of mobile phone and nintendo-type game player car chargers - find one with nominal 3V output - google the switching regulator type - and tweak the circuit to suit.

 

You say 9v to 3v, but did not indicate what your current requirements are.

What is supplying the 9v; a "transistor" battery or a power supply?

 

More info would definitely be a plus for all of us. If it's a low current requirement and you're not worried about losing some amount of power as wasted simply putting the proper wattage 6V zener diode in series is an easy method. Of course this depends on if the "designed for 9V" circuit will even operate on 3V to begin with.

-- or -- do you have a circuit that requires 9V but you want to be able to power it with a 3V source? If so, we'll also need the current requirement, then a proper boost supply circuit can be made.