Good work!
I like how hitting the 'third leg' causes more damage XD, and how, I think, you anticipate what players would likely do and come up with a way to prevent cheesing in your game design (thought I could just camp at the bottom for the yellow boss, for example...nope lol).
Have you thought about just having the shooting being automatic? I find myself just holding down the shoot button anyway, since there are no resources I need to keep track of other than my HP.
Overall, I like the game! Looking forward to the fourth boss!