Selecting partition for backups on Time Machine

I have a external hard drive for Time Machine backups that has three partitions: one for my iMac, one for my MBPro and one for select files.

How can I select the partition that I want for my backups and not have TM backing up to the other three: I connect the external drive to my MBPro and want my MBPro backups on the MBPro partition and not on any of the other partitions.

I appreciate your help and suggestions.

Vernon