Team Fortress 3 will probably come out when TF2 loses all it's hype. It's already losing some, not many good weapons being added, no real updates in a while, 4-5 years old(guess). my guess is that they may consider starting development and having it ready for a possible date around 2015-2017. What it will probably be is just like the other team fortresses, 9 classes, 2 teams, millions of combinations to showcase your charector, etc. probably ill have a promo for having TF2.
Then again, with the main charectors dead(those 2 who died in the comics), those were basically the 2 who made the story line. fighting each other for their companies.