When you disposed of your other two iPads did you wipe the iPads back to "factory setting"? When I had to return my iPad (3rd gen), I had everything all set up including Find My iPad app and registering it on iCloud, etc. The technician that worked with me to replace this iPad with a new one, showed me that he was wiping the iOS of all traces of my iPad and my information. He showed me how to do it in settings. Just to prove to myself that I could not longer see that device, I went home and launched this app on an existing iPad and sure enough, I could no longer find it.
If you can still see your former devices-->that could be good and bad. Good-->you know where those iPads are or bad, in certain cases all of your apps are being used by the new owner...however, this isn't all that alarming since, they have to know your passwords to get access to any password protected information. Another good thing about being able to see those iPads is you can send a message to the new owners telling them to kindly create their own Appleid?