Some of the apps have their own settings, and the s3 has its own general led on/off switch which doesn't seem to really work. Not really sure what's going on.
I was having problems getting Google Voice notifications working. I finally did with the steps found in the link below. However, I am seeing the solid purple/pink LED now that Google Voice is working. Until I find a fix, I've disabled LED notification for Voice.
( S3 Google Voice notification fix found here: Google Groups )