normally you need to buy an adapter at any store to give you 9V~12V DC output. Then online find Switching Regulator, or LDO to buck the 9V~12V to 3V DC.
In more discrete explanation, you need a 220VAC -> transformer, full-wave bridge-rectifier, regulators -> 3VDC
What I can is give u the keywords, then you put some effort to find online, there is your solutions.
Use a transformer to step down the voltage. You can use a transformer that is rated at secondary voltage between 5V and 12V.
Rectify the output to DC using a bridge rectifier and filter the output DC with bulk capacitor. Feed this DC to a voltage regulator. You can use the LM317 regulator. Configure it to give 3V output. You can use any regulator that can provide 3V output.