The standard license permits installing Microthemer on 3 sites in total, not 3 sites at the same time. So it’s not possible to transfer site licenses. This is because it’s a feature of Microthemer that you can completely uninstall it after making style edits, and still have the changes stick. See MTs Preferences > Inactive tab for more information on that. As such, there would be little advantage of the developer license if the standard license wasn’t for 3 sites in total.
However, in the case of dev/staging domains, I’m happy to manually delete those from your quota upon request. Just send me a quick message when you’re ready to go live.