Apple allows you to share and install OS X Mountain Lion on all the supported Mac computers you own. Follow this tutorial on how to do it: Make sure you have enabled file sharing on all the Macs. If you have not go to Apple menu > System Preferences > Sharing > check “File Sharing”. Now, from the finder, go to /Applications/ and find the “Install OS X Mountain Lion.app”.
Open another finder window and hit CMD + K on the keyboard. This will bring up “connect to server”. choose “Browse” and connect to the shared Mac and find the “Install OS X Mountain Lion.app”. Copy this file and run the installer. Simply, was not it? Simply repeat this process on all Mountain Lion supported Mac computer models you own.