A fine, simple game; although I think it is a bit hard and unfriendly to newcomers. You lose when you miss, but you also don't see the angle of the gun before you shoot. That way, you're going to lose a couple of times before you memorise the trajectories of the three poses.
I'd switch to mouse controls. Keep the three poses, but move between them by moving the mouse up and down. And shoot with a left-click. That way, the players would still fast enough to react to incoming enemies, but would also know which way they're going to shoot.

Also, since you don't use the left half of the screen, you could crop it, so the player would stand at the left side and there would be no unused space.

I didn't need the 'middle' shoot – I just waited and used the 'up' shot when the enemy was closer. The enemy coming from the top is thus effectively the hardest one and the most likely reason you'll fail. You can shoot the two enemies in lower layers when they come close to you, but there's only a short window of opportunity to kill the top one. And since the enemies keep getting faster, the window gets smaller and smaller.
I would try to balance the enemies and make them equally hard – e.g. make the top enemy slower and the bottom enemy faster.

Oh, and if you add +38 to your vertical resolution (to compensate for the WebGL footer) in Newgrounds Project Settings, the game window will stop being 'cut off' at the top and the bottom. Or use 'Minimal' WebGL Template if you don't need the fullscreen button.

It's a simple game, really, so there's not much to talk about. If you kept the same screen size as now and moved the player to the left, it might give you more space to add things like different enemies, collectables/power-up etc. – but then it also would be harder to properly aim and shoot, if there were still only three fixed poses.
For a game jam entry, this is good, but if you wanted to spend more time with it, you could improve it further.