If the card you sacrifice isn't still in the graveyard as the first ability resolves, it won't be exiled. It won't be a card exiled with Bag of Devouring for the third ability.

Bag of Devouring's first ability is a triggered ability that happens when you sacrifice permanents, but you can't just sacrifice permanents whenever you want. You need to have some other means to do so, such as activating Bag of Devouring's second ability.