[dnanian]

You can't boot over the network. If you partition the drive into (at least) two volumes, one for the bootable backup (for the computer directly attached) and another to store the images files for the other two (see "Backing up over a network" in the User's Guide), you'll be able to back up all three on the drive, but only the "direct" one will be bootable.

You'd be better off with three drives, really: I always prefer a direct backup to an image. But given the above it'll work.

Note, though: SD!'s license is not a "family" license. If three people use those three Macs, you need three licenses.