M.F.
Sounds familiar. I too have a very talkative 3 yo. He's a firstborn, and i just think that's the way of it. We do little contests on who can be the quietest (guess who wins). I send him into another room (his own to play or the den with the train table) for a few minutes peace, but he takes it as a punishment rather than a mommy need, so I don't send him off too much. Just take a deep breath and leave the room when it gets too much. My son still knocks on the door, but at least there's a buffer ;)
M.