I tested 3 player matches with some friends and here are my thoughts.
It was still fun. Games are longer and grindier, which I find fun. There are usually a bunch of level 3 monsters on the field making the endings of games really explosive. It's very possible to kill 2 players in one turn.
Games are far more defensive. Since you can only have 3 stacks attacking is much riskier. Exhausting even one monster can cost you the game since you have to survive 2 rounds of potential attackers.
It might be fun to allow a 4th stack so your resources aren't so strained on your opponents' turns.