I have three iMacs - do i have to buy a copy of mountain lion for each?

No. It can be install on up to 5 Mac in a household.

Allan

You'll have to save the install file to a thumb drive before you install and use it on all three Macs or logon to the App Store with your AppleID on each Mac to install.

What Russa is saying is that if you don't save "Install OS X Mountain Lion.app" before you install you'll need to download it again each time. It's important to save it before you install because it self-destructs after restart.