Backup behavior on multiple computers

WD External Drives WD Portable Drives
3

I have had my drive for some time to back up one machine. I now need to back up 3 machines. Is this possible please and, if so, how do I go about it? When I backup, the options are to do a complete backup or an incremental one. I am worried that if I connect a different machine, it will overwrite my existing backups. The ref no. on the back of the drive is WD3200D032-000.