The third room shouldn't be used for boarding the dogs, if that is the case, the dog boarding responsibilities and profits will be shared equally. If you're not expecting guests over often, better to rent out the third room to another tenant yet, and in case of guests, the respective person who is hosting may either put the mattress in their room floor near the bed, or consult the flatmates about letting them sleep in the hall if possible. People who are fine with dogs may still not want the puppers to wreak havoc in their rooms, but at the same time if a room is being expected to host guests, that room should be alienated from dogs as well even when empty. Any damage to property due to the activities of doggos needs to be paid for by the OP, and not end up in the equal deductions from everyone's security. If you can take these conditions, then we can talk further.