Hello,
Thanks for trying the game, i'm glad you enjoyed it so far.
The bug you show is caused by a memory leak in the game related to the damage numbers in the game. this happens a lot more by fighting bosses cause there's 25 dragons fighting at once. I recommend reloading the web page or even closing down the browser you use and reload the game, if this still doesn't help please describe what actually happens when you press the boss battle button and i'll look into it, I have yet to pinpoint this bug and wasn't able to remove it yet. also, if you have another browser please try that one too. it gives me more detailed overview of what might have gone wrong.
3 dragons in battle should be fine, this is what the game is balanced around, although it might be a bit harder at start than when you have better dragons.
I'm adding a reset skill button option so you don't feel pressured into anything.
Implementing an "inventory" for spells should be doable, but it might take some time.
I'm working on some bug fixes atm, and will implement what I can by a patch soon.
Thanks for your feedback, it means a lot to me!