Of course it is possible! There are many tutorials on the internet that discussing this topic
Search by writing this "unlock galaxy s3 virgin"

I have given serious consideration to buying a Samsung SIII from virgin with the hope that it can later be unlocked and used on another network like T-Mobile. I haven't been able to get a clear answer whether this is possible or not, but my main concern is that I would still be able to get 4G from another company.