As long as they're all logged into the Mac App store with the same Apple ID, you can re-download the installer to all the Macs.
Or you can save a copy of the installer file before you run it and use that to update the other Macs. The ML installer file will appear in your Applications folder when it's done downloading. Copy it to another location (don't move it).
If you don't copy it before you upgrade, it will disappear when ML is finished installing.