Well the demon heart is definitely in the drop list, you can be sure about that (like 1 or 2%, definitely not lower, as the droplist engine only allows 'rounded' percentages). All 3rd dungeon bosses can drop it (except the first one, if I remember correctly). Unless there is a serious bug about that (that possibility exists; after all, I never tested dungeon 3 by myself and did all the balancing and coding based on player's oppinions), it will drop eventually.