In acts 2-4 there are 6 dungeons in the BF, rather than 3. Assuming you found the level 1 and 2 keys, then the dungeons you uncovered may have been 3, 4, 5 or 6 (for act 2, the level 4 dungeon is to the north, dungeons 1, 2, 3 and 5 are near the southern edge, in order going east, while the level 6 dungeon is to the far east).
Because each level BF key corresponds to a particular dungeon, it makes sense to order the dungeons by level (I think the opponents tend to get stronger in the higher level dungeons, as well).
The level 1 BF key opens the level 1 dungeon.
The level 2 BF key opens the level 1 and 2 dungeons
The level 3 BF key opens the level 1, 2 and 3 dungeons
etc
