I have three Macs I want to upgrade to Mavericks. Is there a way of re-using the install files, so I don't have to do three separate 5gb downloads?
Download the update from the App Store. Once the file has finished downloading, check your Applications folder for the "Install OS X Mavericks" icon. Copy this installer to your other computers before going through the install process. The installer will delete itself after the install is finished.
Background: when you run the "Install OS X Mavericks" application to upgrade your Mac, the app deletes itself when it's done.
Solution: make a copy before you run it. Since the App Store opens it automatically, you need to quit it, go to the /Applications folder, and make a safe copy somewhere. Actually, my standard procedure is to create a zip archive of it (select it in Finder, then choose File menu > Compress). The zip archive won't get deleted, and is easier to save, copy around, etc than the application (which is actually a folder with complex internal structure). Anytime you want to reuse it, copy the archive to /Applications, double-click to expand, then double-click the expanded app to run it.
There is one hangup, though: the zip archive will be over 5GB, and the FAT32 volume format (frequently used for flash disks and such) can't deal with files over 4GB. You may have to reformat disks to a more modern format, like exFAT or Mac OS Extended, in order to use them to store and transport the file.
Another option is to use the installer to convert a USB disk into a bootable (and reusable) install disk. This makes reinstalls much faster since the installer is stand-alone and bootable.
Either use DiskMaker X, or do it yourself at the command line with the createinstallmedia tool hidden inside the installer application -- right-click the installer app, select Show Package Contents, then open Contents/Resources, drag the createinstallmedia tool into a Terminal window, and follow its instructions. You should end up with a command like this (assuming the disk you want to erase & convert into an installer is named "Untitled"):
sudo "/Applications/Install OS X Mavericks.app/Contents/Resources/createinstallmedia" --volume "/Volumes/Untitled" --applicationpath "/Applications/Install OS X Mavericks.app"
DiskMaker X takes a bit of time for a USB, and beside the initial screen you will think it hangs, but it works, just look on the Led of your USB stick. In my case to revert from Yosemite (I do not tell you what I think about Yosemite and Apple Inc.) to Mavericks it took more or less 15 minutes to generate the installation on a USB 3.0.