As stated, are you asking for a power supply to power a 3s lipo charger or are you thinking of using a 12v source to charge a 3s lipo battery. If you are plugging the lipo into a 12v power supply, I think you will let the magic smoke out real soon.

I have several PC power supplies I have converted to power my chargers. It cost about $10.00 to convert (2) power supplies.

There are a lot of budget priced chargers that are AC/DC.

Buzz.