Good job on your first game! Not bad for being made in 3 hours! The basic movement and shooting mechanics work well!
I would recommend adjusting the difficulty by reducing the number of enemies or make it so that not all of them "home" onto you. Would also recommend just holding down the mouse button to fire. Wasn't able to get far enough to buy any of the better weapons but that feature looks cool as well. I'd say you got the basics of a good top-down shooting game!