I'm glad it helped. I guess Spotify is such a massive organisation that it can't be on top of everything all the time, but simple things like this, as you say, should be made clear in the FAQ section. Something so simple, yet it's causing so much unnecessary frustration.
Re: Username or Password incorrect (Error code: 3)
Holy heck that is some boneheaded, unintuitive software development. Who are these people who program things like this? Not a single indication that I would need to enter anything other than my email address, and yet this solved my problem.