The Twins are not back in T3.This could mean they make no appearance whatsoever to having only a brief cameo appearance. Who knows (which is part of the speculating fun) but this will have to put the Twins out of the Transformers cast list until new verifiable information contradicts it since this is as official as a source can get (but one that likes to have fun with fans). Consider the waters officially muddied and don't see it getting any clearer any time soon.
Update: Just a brief update on why don't entirely take Michael Bay at his word. While Transformers 2 was in post-production, he told Empire Magazine print edition, "No, Megatron isn't in the sequel" but several months later confirmed he was. He also indicated during TF2 production a desire to engage in a "misinformation campaign". I see no reason why he would not continue to do that for TF3. It just means that even though he is an official source, his information cannot be taken as gospel; just as something to fuel the speculation like any other non-official source.