I’m quite new on the AAM world but In case of last auth + Device graph, if we have a device that never logged in do we consider this device ID on the segment qualification ? Or all anonymous are not take into account on this PMR ?
Sorry if I am not understand your question correctly.
So basically AAM system will be captured both Anonymous and Authenticated (Logged In) users into their data base, however when you come to segments as you mentioned it may be varyaccording to the profile merge rule conditions.
According to your use case, If you want to segment the "Anonymous" (No Logged In) user into one set better we need to choose "No Authenticated + Current Device" to segment only if the user is not logged in across.
Also, if you want to segment the user who logged out from last three device then you need to be go with "Last Authenticated + Device Graph".