3" tile on a 42" vanity, just need to do the countertops, factory built sink going in on tile top. Should I use some sort of membrane to waterproof? Can I just build the sub out of plywood and cement board? Please help!! Need some ideas on getting this done. thanks

Hi
Plywood and cement board will work just fine. The only area you might want to waterproof is the cut out edge of the sink hole, but it's not really needed.

thanks so much for the info, we'll see how it comes out.