Impossible to say at this point. As others have mentioned, Sony have four major conferences throughout the year where they need to announce new content to get people excited, and Nintendo has regular Directs where they announce new games. In comparison, MS only have E3 and Gamescom, so they can afford to throw everything they have at those two events. That doesn't mean they will have nothing else throughout the year, as their announcements of Forza Horizon 2 and Forza 6 prove, but it is an advantage nonetheless if you are simply comparing E3 shows.
Nintendo are hardest to predict in terms of new games I think, there seem to be less rumours around their upcoming games, unless I have missed them. Maybe a new Metroid, maybe a re-reveal of Fire Emblem X SMT, hopefully some cool new IP like Splatoon.
Microsoft will have a Gears announcement, whether that is a remastered trilogy or the new game in the series. Perhaps both, with a short teaser for the latter. Maybe Battletoads and/or Banjo will be there, and they will have a couple of third party exclusives. The new studios MS created are all apparently making HoloLens games, which is disappointing for me, and I doubt we will see any of them at E3. More info on Scalebound, Crackdown 3 and QB will be welcomed.
Sony are rumoured to have GG's new RPG Horizon, a JRPG from Sony Japan, Media Molecule's new IP, Quantic Dream's new IP, Sony Bend's open world horror game, a new Sly Cooper game, and colloborations with Sega and Level 5 in the works, and a selection of those will no doubt be present among a few surprises. New info on Street Fighter V, God of War IV, Drawn to Death and WILD is also likely.
Looking pretty good overall. I couldn't pick a winner at this stage