I work with 4-mm plywood, don't know anything about 3-ml. I tape the inside seams to keep the weight down. You can get away with only epoxy on the outside but you will have no abrasion protection. I remember reading a post by someone here years ago who had a kayak with absolutely no glass at all, just glued together. He said that it lasted 20 years with no issues. Apparently it was still in use at the time of posting. 3-ml seems kind of thin for a kayak with no glass on the outside. If you're gonna go that thin, I'd say glass the entire inside instead of just using tapes. Otherwise, I think a 4-ml ply with inside tapes would work just fine...but don't hit any rocks.

Robert N Pruden