Bethesda is under no legal obligation to make this exclusive unless the agreement they had has conditions that directly exclude Hearthfire. If no such conditions exist, it counts.
2 may be in 3, but 3 is not in 2, and Bethesda is under no obligation to provide the additional 1 when the legally-binding agreement only specifies 2. If they choose to do it, it is their own choice.
2, 3, 3,000,000 it's irrelevant. The point is how it's interpenetrated. If I tell you your first two cups of coffee are free, I'm under no obligation to give you a third for free but you sure as hell are going to
expect the third for free. If Bethesda release Dragonborn for the Xbox first then they purposefully and willfully mislead the PC and PS3 users regardless of what they legally meant.