Hi Nithin,
It is normal for a power supply output voltage to drop a little when a load is connected, unless it is a regulated supply., in which case the drop will be tiny.
A large voltage drop from a power source under load generally indicates that the load is too great for the PSU's output power.
If you put a low powered 3V bulb across a normal 3V battery, the voltage drop will be small.
But if you put a high powered 3V bulb across the same 3V battery, the current will be greater, so the voltage drop will be larger.
Post your circuit and tell us how you think it should work, then we will have something to work on.
