Take another look at rule 20.2(c). It says:
"The hailed boat shall respond either by tacking as soon as possible, or by immediately replying "You tack" and then giving the hailing boat room to tack and avoid her."

In this scenerio, Boat 2 satisfied rule 20.2(c) by tacking. She is not required to give Boat 1 room to avoid her. In fact, if both boats had tried to duck Boat 3, Boat 1 might have been required to give Boat 2 room to avoid Boat 3 under rule 19.2(b).

Regards,
Eric