I got a confusing notice today too. It was for the amount we got for the second stimulus in January. We think it's just a late notice for that one.
Yes, I’d be surprised if IRS notices for the 3rd stimulus have already been sent out (although not entirely shocked, as I’m already amazed and impressed that people are already receiving the 3rd payments themselves).
Maluku, can you double-check the title of the letter you received. It should say something like “Your Third Economic Impact Payment”. If it in fact says
second payment, then I think it is as Morning Glory describes.