Director Michael Bay replied to some rumor on his official site’s forum and he revealed that the first trailer of Transformers 3 Dark of the Moon will show in front of Narnia 3:
“[…] the Transformers announcement piece comes out on Tron and Narnia.”
Director Michael bay
So the first trailer of Transformers 3 will released on December 10 with Narnia 3, or maybe a few days later, on December 17 in front of Tron legacy.
Anyway, I guess the movie trailer of Transformers 3 Dark of the Moon will show up online soon after if not before.