Nedarim 45: The Smallest Public

Closing chapter 4. Retracting a statement of hefker is not effective when the declaration of hefker took place before 3 people - then it's automatically ownerless (and according to biblical law, one witness would be enough for immediate effect, but the sages required 3). Also, chapter 5: where jointly owned property may be subject to vows swearing off benefit, and the question of whether that shared ownership means one must be stringent with regard to the potential benefit, or whether one can assume that the shared ownership means there's no real way to lose the benefit.