Munchkinomicon...

[Clipper]

The rules say: "Sacrifices three cards. You may do this at any time, even in combat, to lure the Munchkinomicon to you."

So there is no restriction on timing. The cards could be in your hand or in play. There's no restriction on the type of cards.

The only time you need to reduce your hand size is immediately before the end of your turn. Your turn doesn't end immediately after you finish a combat, so you have time to play cards, lure the Munchkinomicon or do otherwise without having to consider Charity yet. If you wait until after giving Charity, then it is the next player's turn already, but that is an OK time to sacrifice your three cards too, as the Munchkinomicon can be lured to you on anybody's turn.