Beware. We ran into this issue while trying to document a different issue.
Apparently the three devices counts any device which was logged in in the past and never explicitly logged out. My wife had two devices currently in use, her laptop on which she could not sign in, and her phone on which she had closed the app. But she had a prior laptop and tablet, on both of which she had used Dropbox before. Opening the Dropbox app on her phone, she then explicitly signed out there, THEN she could sign in on her laptop.
Then it gets interesting. On sign-in, she was plopped into the setup sequence, even though she had used Dropbox on that laptop just days before. There appeared to be no way to escape it, so we went through. After that the original issue was gone!
