I personally own 3 MACs (2 for me, 1 for my daughter). Can I purchase a single license and install OmniGraffle on these machines?

[whpalmer4]

You don't mention whether your daughter lives in the same household or not. If she lives with you, you should be okay, but otherwise I think you might be skirting the intent of the license.

My reading of the Omni license agreement is that you can install on as many of your machines as you like, but buying only 1 license means that only 1 machine at a time should run the application (and the applications will enforce this on your local network).

Here's some text from the "About our software licenses" page in their online storefront:

About our software licenses

Our products are usually licensed for a given number of computers or users, whichever is lower. For instance, one person with both a laptop and a desktop computer (or, say, a home machine and an office machine) can legally install and use a single license on both computers, though not at the same time. Similarly, a home computer shared by three family members needs only a single license of the software.

However, the licenses are not "floating" licenses. For instance, a business with twenty users needing to use the software across twenty computers needs to purchase twenty software licenses, even if no more than ten users are ever to use the software simultaneously. While the licensing software cannot enforce this restriction, this is the intent of the license, and the understanding under which the licenses are sold.

[Brian]

When purchasing licenses for our applications, you need enough licenses to cover the number of machines or the number of users, whichever is smaller. Sounds like you need two licenses in this case.

Since you and your daughter share a household, you'll want to purchase a family pack for OmniFocus - that costs less than two individual licenses, and can be shared between you and your daughter.