CVG (Computer & Video Games magazine) has speculated that DX3 might be a prequel, based on a date shown briefly in the teaser trailer. I hope it is, because the further the story moves into the future (I think DX:IW was around 2070) the less grounded in reailty it will be, and the less realstic the setting/gameplay will be. I don't want to see crazy mechs and teleporters and all that far-futuristic sci-fi baggage.
As an aside, I've started playing Invisible War and so far I'm about 3 hours into it. Not bad, not particularly good either. There's definitely some good stuff in there, but it's buried beneath a big pile of poorly-designed, poorly-implemented crap. My main complaints so far are that the levels are tiny (making loading a pain) and that the whole game somehow feels loose and floaty, probably due to its console origins.