scott once you have purchased it once it can be then downloaded and used on all your devices as long as they are all on your Apple account so you can freely go through the install process and buy it again it just won’t charge you , I know, it’s not a great way Apple do it as you never know clearly if you will be charged again , they should do it differently so it’s clear, but they obviously don’t know how or it’s too hard to program, I don’t like fuzzy logic but there seems to be plenty of it now days.
The good thing with ST3 it’s worth the money anyway even if you paid for it for each individual device.