I have an iMac and two Mac books. Can I purchase only one license of the new OS X Lion and install it on all three computers? ... If so, what are the guidelines installing the OS on multiple computers?
You only need to purchase Lion once to run it on all of your computers. When you download it from the App Store, there should be an "Install Mac OS X Lion" app in your Applications folder that you can copy to your other computers. You may also be able to access it from the App Store on your other computers using the Purchased tab.
This is a screenshot of the License Agreement: