I was having an issue with installing the 3.0 Beta on my non-4K Firesticks. It looks like Amazon now has the 3.0 stable release out and I was able to install it, but it won't load. I get the "Preparing your TV..." splash screen and then it returns back to the previous/home screen. I get the same behavior on both of the non-4K Firesticks I have. The two 4K Firesticks I have give me no issues, they load and work great.
I've tried a factory reset of the Firestick, but it made no difference. I've looked for any sort of a log to perhaps show me the culprit, but I guess there's no logging available to the user on a Firestick? I tired installing version 2.1.27, but it's not loading either - and it doesn't even get to the "Preparing your TV..." splash, just a spinning loading circle.
It's running Fire OS 126.96.36.199, which it says is up to date.