If you think your Samsung Galaxy S3 is losing its charge, buy a new battery. I bought a battery, a Rayovac, at Batteries Plus.
What was happening with me was that I was charging my phone in the middle of the day. Also, when I was charging the phone, the charging was long.
Installed the new battery and it charged right up.
Note: Rayovac has a high capacity battery for the Samsung Galaxy S3, however, it is a thicker battery. As a result, you need a special back plate to hold the battery. The back plate that came with the phone will not hold the high capacity battery.