Well, I'd first suggest a software restart via the menu system. Then a hard restart (which is nearly the same thing but sometimes does things the software restart doesn't). You've tried that, so the next step would be a factory reset -- which you can also do via the menu system. But I think I'd contact Kindle CS before going there. It WILL remove all your content and personalization so it's not a step to be taken lightly.
There are links for Kindle Customer Service, by phone or email, in the FAQ sticky at the top of this section of the Board.
If it's under warranty still, they'll replace it for you if they can't help you work through a fix. Even if it's out of warranty, they might be able to help -- either replacing it at no charge, or giving a discount on a purchased replacement.
If there's one thing that's even better than obscure problems like this, it's obscure problems like this that suddenly go away!
I don't have my K3 any more, the only thing I can think of is... if you hold down the troublesome button (or press it quickly twice), does it then do "back"? If so, then it could have been the switch sticking. We have heard of this in the past (and I've had it), but not with the "back" results.
I have had my K3 button do funny things at one point and it turned out that I had something stuck in there. Probably some cat dander or snack crumbs. . I blew it out with some spray air and it was fine after that. But that was a mechanical issue.
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