It's fun! I somehow miss a win/lose scenario.
Maybe give the player some life (ships attacking you) or win when all are upgraded at 3 start.
I would love to see an increase in recharge rate with upgraded cannons or a lower ammo consumption, I had the feeling that recharging three upgraded cannons is nearly impossible and just upgrading and recharging two cannons is absolutely enough to destroy 90% of the enemies.
I like the idea, it's fun to try keep all guns charged :)