Agreed about doing a little more back work.
It's possible to do 3 upper body days but they don't necessarily have to be the same. For example you could base one off bench/row, the second off overhead press/rear delts, the third off incline/pulldown, that kind of thing.
Another thing you can do in your situation (happy with legs, want a little extra upper body emphasis) is one setup that comes up here from time to time: chest+back+delts/legs+arms/chest+back+delts. It works well for its purpose, and nice to wrap things up in 3 days for that matter.