I played a three player game a few times and found it to be a little frustrating in terms of initiating the fight. In a 1v1 game, you want to be the one to draw first blood, as that sets up the momentum of the game. But in a 1v1v1 game, I feel like you're punished for starting the fighting as you become the target of retaliation, and while you and one mage kill each other, the third one just sits back and then cherry picks the win. Three player free for alls take longer because of this.
Also tried a variant with each mage being assigned a target, and victory was only achieved if your target died. This resulted in a paradox of not being able to retaliate against the guy trying to kill you, as hurting him only helped the guy you yourself were trying to kill.
Three mages on the board, regardless of board size is just an off kilter style of play. I feel like the best option is to have one player control two mages and the other two work together in a 2v2 game.