Oh good to know there is more! I will say that when I got to the second area, I didn't realize there was a way to go up, so some of the transitions between areas was not always clear, so that might be why I had trouble finding the other two bosses. If I recall, it was a line that looked like it would be a wall/ barrier past a root at the top of the screen in the second area.
Does a secret open up / unlock when you defeat all the bosses??