This is with a mono-sphere deck. If I start with less than three heroes in play at the start, what do I do with the extra hero(s)? They have no cost, so I assume they don't get mixed into my deck. Are they just put aside and are never available?
Where to put heroes that don't start in play?
Correct. At the beginning of each game, every player selects 1-3 heroes to use during that game. Any other heroes are just left with the rest of your unused cards, in your game box or wherever you decide to store them and play no role in the game whatsoever.
Makes sense, thanks for the clarification 