Have you:
--dropped the phone recently?
--gotten the phone wet?
--left it on the dashboard of your car in the hot sun?
--otherwise abused, flogged, mistreated or otherwise lowered your S3's self esteem?
If so (and we've all been there...) the camera may have been physically damaged, and may not work any more. Do you have any kind of insurance or a replacement plan with your carrier?
If not, have you:
--tried to reboot the phone? (turn the phone completely off, then turn it back on)
--checked with your provider's support website to see if you have the most recent software version installed?
--done a factory reset of the device, if the other two suggestions have not worked?
Welcome to AF! Keep posting here with what you have done/not done to fix it, and we'll try to straighten it out for you. Be sure to join the forum and become a part of our community.