I've read and seen a couple of interviews in which they claim season 3 will start shooting the first episode in December, before Christmas.
I found this interview to to Sam Trammel:

VAN SUSTEREN: So you got picked up about a month ago for the third season. When do you actually start shooting the third season?

TRAMMELL: I think we will start shooting in December. We will try to get one in before the New Year starts.

We had some things happen this year, and we were sort of -- we almost didn't finish some of the episodes right before they were aired. So I think he wants to get more of a head start this year. So I think we'll start in December.

And one to Alan Ball:
"There is no time for Alan to take on any other projects because he and his team of writers will start arching out season 3 while still in post-production of season 2 and he is planning to shoot the first episode prior to the Christmas. So they will be a little bit ahead of the game next year, because right now they are scrambling to get all the episodes done by the time they air."

The AB interview is from June and the Sam Trammel is from August. So if everything goes just as planned, they should start filiming anytime in the following weeks if they want to finish before Christmas.

I WANT SEASON 3 SO MUCH!!! I'm addicted...