Correct -- fighting the boss is one long turn consisting of several rounds. So you can drink for 3 at the start of the boss fight, but only for 2 after it starts.
Hmm I'm confused.. on the Boss Fight card, it says "You can use your potions during each boss round. The start of a round is the same as the start of a turn for a Healing Potion."
So doesn't that mean you can heal the same as if it were a new turn, meaning it heals 3? I think I must be misunderstanding something
(Super fun game by the way, just got it last night and have played a good handful of games already)