How hard would it be to use unity3d to make a 2D match-3 game?
Just as easy as in any other environment.
Unity provides a lot of tools, but most of them (except for GUI and graphics tools) don’t help much in puzzle games, but then nothing really does (on account of the very varied nature of puzzle games).
An aweful lot easier than trying to do it in c++ and OpenGl for example.
About as hard as doing it in Flash. Which is to say that it’d be as easy as it gets (unless there’s a product out there devoted to Match 3 games)