Sorry I'm not an expert in this, but while googling I found apk of google play store, I thought I could perhaps reinstall using that?

Yea I did, clear data and clear cache of play store, still couldn't work. The account should be correct, my contacts sync to gmail is correctly displayed in contacts list. Also if I browse google play website, they mention that certain apps is compatible with my old phone (xperia X10i), so I think the account log in is working.

In any case a clean factory reset sounds a better option, I think I'll do that. Luckily I can download titanium backup free apk without needing play store.

Thanks for your help bro.

guys can you all check yours s3 whether got stock video or not ?
me just now check don't have @@
but my friend said have, any1 have ?
if got can share with me ? want to download it >.<