If you want to use the three machines under the same single user license then you will have to make sure you are only signed in to two of them at a time.. It might have to become a habit for you to always sign out when you are done using a machine so that you have the extra sign in available.
Thank you so much, Ned. It's not so much that I don't want to pay extra, it's that as a mixed media artist I don't use digital for everything, so your answer is really fantastic! I can easily learn to deactivate the least used all the time.
One more question, though. Will Adobe penalise me for doing this? Or is it an acceptable thing to do as per their rules?
Despite my last smaller question, I'm very grateful for your assistance.
I have never heard of any penalties for using the software features. As far as I know the intention of their sign-in/sign-out scenario is to allow you to change machines as needed. They used to have a lifetime limit for activations with licensed versions but they did away with that some years ago.