Rules and Errata

Not that I expect anyone is confused by the card, but as written, Compelling Offer has you keeping all 3 cards if you reject the difficulty increase. You are never instructed to banish unchosen boons in that case, which is clearly not the intention.

Compelling Offer: When you encounter a villain, choose a type of boon other than loot and draw 3 non-Basic cards of that type from the box. You may choose 1 and put it under this card and banish any not chosen; if you do, the difficulty of checks to defeat the villain is increased by 1d20. If the villain is defeated, add the boon to the scenario rewards; otherwise, banish the boon.