Unfortunately additional information is needed to properly answer your questions. Generally speaking, a third party cannot alter, modify, or relinquish the rights of another without their specific authorization. However, some agreements have certain language and provisions that allow just that. For example, a continuing guarantee would be valid as against all original guarantors even if only one of them renewed, or extended the agreement (in this case a lease). Also, if the parties were in breach of the agreement (i.e. behind on rent) the original lease may have terminated on its own terms and then a new agreement could be established with only one of the three original parties.