This is probably not a physics question, but anyhow ...
Any modern phone will not allow it's battery to be overcharged so leaving it on the charge won't overcharge it. However some phones will cut the charging at 100% then resume it at around 90%. You may find when you disconnect the charger that the battery is somewhere between 90 and 100% charged. I don't have an S3 so I don't know if the S3 does this. If it does your phoine would be continually cycling between 90% and 100%, which I suppose could shorten battery life, though I'm not sure it would be worse than removing the charger and letting it fall to a lower level.
There is an article on battery life here, and this is an interesting read though I'm not sure it sheds much light on your specific question.