But does dragons + 2 mana = 2 or 3 buffs?

As far as I can tell from my balancing spreadsheet, play testing, and general intuition - yes, it does.

That being said, obviously we don't always get things 100% right all of the time, and we'll be keeping a close eye on win rates and usage of cards as we always do and make changes if necessary, but to the best of my ability I do think this will be pretty well balanced overall while also allowing the cards to feel "Legendary" and exciting to get and play in battles.