I don't think the DAG size has reached 3Gb yet so you should still be able to mine ETH with a 3GB 7950, but you would probably get better results from mining some other Ethash coin, according to this post:
https://bitcointalk.org/index.php?topic=3050943.msg32930373#msg32930373

This site used to give current DAG size for various coins, but it's down at the moment:
https://investoon.com/tools/dag_size

Here is a link to an archive of the site from 28th May.
https://web.archive.org/web/20180526100812/https://investoon.com/tools/dag_size