There DOES appear to be a limit of 3 per account.
I just counted the number on my account, and there's ten devices listed. Only one of them is not in use, so I have 9 active devices. I've had more than three for many, many years. That is not the limit.
Why you had issues, I have no idea. Some users have reported successful activation by using a phone hotspot to do the initial setup, then switching to the home network. I know, that doesn't make any sense. But it's worked for many users, so it's always worth a try.
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