please help me in constructing a 0-30 volts, 3 amperes variable power supply....
I am currently using a 30-0-30 volts center tapped transformer... I really need help because our instructor wanted us to make use of transistors instead of ICs... please do help me....

 

Two online sources which may help:

Linear and Switching Voltage from National Semiconductor

and

the first chapter of Linear & Switching Voltage Regulator Handbook from On Semiconductor

These cover feedback amplifer theory, reference voltage techniques, and how pass devices are controlled.