Will I be able to mine zcash for life long with 3gb GPUs?
Yes, as long as the mining parameters for equihash do not get changed the memory requirements stay constant.
So how long can I mine zcash using gtx 1060 3gb cards.
i guess you will be able to keep on mining with them, i dont think there is anything like dag files on zcash.
however... if 3gb is not going to be profitable enough anymore on ethereum based coins, then they will switch at a certain moment to zcash, which will let the difficulty grow more. (if you don't have choice left, it's better than nothing). anyway, as long as it it profitable, no reason to stop mining with it, right?