Why can't my laptop detect my S3 phone?
Samsung has very bad connections on that line of phones (S3, N2) I know this because I have a Galaxy Note 2 and about 6 months ago it was doing the same thing, then eventually it stopped charging all together. I also have a friend with S3 and the exact same thing had happened, except it wouldn't charge(luckily his was still in warranty).

You need to check if it is being detected in another PC, if not, the connection is broken.If you really really need to be able to plug it into a PC, you will have to buy a replacement charger dock port.

Or you can use airdroid to wirelessly transfer files to an from your PC over a wireless network.