It's been confirmed by Orci that he and Kurtzman are not coming back to TF3 - Here is a hint to why:

Quote Originally Posted by Originally Posted by relentless @ donmurphy.net
i know in the creative process, especially with someone as strongheaded as mr bay, there were things you were came to blows with scriptwise. im hoping ehren fights valiantly for making the humor a bit less crude, one of the oddest things about revenge of the fallen were how many parents I heard wouldnt take their children to see it.
Quote Originally Posted by Roberto Orci
I can't blame them.