[AC Question]

I swapped batteries with my wife's s3 and same thing happened but my battery in her phone was OK.

Conclusions -
1. It is not a battery or charger problem.
2. It is not an app problem as battery drains when phone is off.
3. The only thing left is hardware. So maybe there is a partial short in a component somewhere.

Solution: I upgraded to a refurbished S5 as hardware repairs on an old S3 are uneconomic. However I intend to look inside the S3 myself to see if I can spot anything.

I suggest that others also check to see if battery drains fast when phone is turned off completely.

[belodion]

Yes, draining while powered off suggests a short circuit or some other fault.

[Rukbat]

No need to post the same thing 3 times in the same minute. IF someone can answer your question (and this wasn't a question), they'll get to it when they get to it. This is a site frequented by other users like yourself, it's not a paid repair site.