Just three maps are not enough for a full-fledged tournament:
I'm just thinking from the perspective of a newbie random player who suddenly wants to jump into the competitive scene. If there's only one map, it's okay, I'll study it, play a few games, and I'll be ready. If there's a pool of three simple maps, it's okay, I'll take a look, I'll think about it. If there's a pool of more than 5 complex maps, like LoM and others, which all need to be studied (and preferably practiced), and then pay money and play against Horn or Expert - no thanks. Hence the logic behind the map selection. I am convinced that the strategy of "looking at a map in the editor for an hour and then playing" in 2017 does not work for competitive play.