Great boss fights! Since they had interesting patterns I want to detail a bit more what I liked and disliked in each.
1) Frog : I liked how the jump makes you bounce upwards if you don't jump, which seems not to matter until the frog starts chaining jumps. The tongue and vomit attacks had very similar telegraphies, and it was hard to see which one would come out during the fight because the frog's designs is so "simple". It seemed unfair to me because these attacks need to be evaded in different ways usually although I get the point to design it this way, the telegraphy just needs to be clearer. Also might be me but I took a number of hits just getting close to the frog cause the sword's hitbox seemed small and the frog's hitbox didn't seem as big.
2)Eagle : this one was a bit less fun because of the character controls and the time it took. Compared to the first boss where you never feel like you're waiting, this one feels more tedious and less hard, it's simply annoying to die close to the end and have to redo it whole... I would have loved it if the frog's dps was slightly higher. The attacks worked well.
3)Dragon : this felt like the easiest of the 3 bosses to me, but might be just cause I got used to the game. I liked the claws and the basic flame breath, and I didn't like when he starts attacking from the side of the screen all the time and I need to wait again and again.
Also it may be just me but I feel your description of easy mode is a bit condescending, just a matter of phrasing (if you want your game to be challenging that much just take out the easy mode and be done with it).
Musics, art and artstyle were great.