Hi @Mary, it depends how serious you want to be about your battery.
All batteries have a limited number of charge cycles, but Lithium-Ion batteries like the one found in your S3 do not suffer from ''battery memory''. This means there is no need to fully discharge and fully charge them. In fact, they suffer from low voltage problems, so completely draining your battery is a bad idea.
They do have a ''cut-off switch'' though, so you can leave them charging overnight. Just avoid direct sunlight, hot surfaces, liquids, dust etc. Of course, depending where you look, you can also find evidence that Li-Ion batteries DO have the memory effect. It really just depends who you want to believe.
Unfortunately, as soon as Li-Ion batteries leave the warehouse they start getting worse, slowly but surely. You just have to accept that and be prepared to replace them after a year or two, depending on your usage and luck. The best approach is to always charge them before they drop below 20% or so, to avoid low voltage and just charge them whenever you need to. Then when things get really bad you can try to recalibrate them or simply pick up a new one by a reputable manufacturer.
Best of luck!