Head on over to the Dev Center if you are a registered user. No news on what has changed in Beta 3 yet. I hope that it is a lot better than beta 2. I had to downgrade back to Beta 1 as beta 2 was too buggy for my liking.
If you still would like to beta test the iPhone OS 4.0 we still have some spots available.
UPDATE: Looks like they pulled it. For those of you that got it: Are you having any problems? I downloaded it on my home computer. I wonder if I should install it. We will keep you updated with the details.