Nintendo
Focus will be the NX of course, hopefully they can gather some decent support for launch besides the obvious port of Zelda U. The BG&E2 rumor would be nice for them if true. DQXI also on NX would be cool for them. I don't have any faith in western publishers supporting them seriously besides some ports of PS4/X1 titles.
Microsoft
I dunno, there seems to be a lot of confusion in the company right now.
They have basically acknowledged internally that the Xbox One won't get the results they were hoping for and they are transitioning to an other business model.
Surely they are working on some hardware revisions but announcements are unlikely to be made in June if the actual products will be released in November since that would kill their business in the meantime.
On the software side I guess that Gears of War 4 will be their main title like Halo 5 last year. A new Forza, Halo Wars 2, the pirates game by Rare, ReCore will also get the spotlight. A couple of new games will be announced. Third party demos for those titles where they got the marketing rights will be at the conference as well.
Sony
I think that a good chunk of it will suck honestly
They have to show their love and respect for their unprecedented partnership with Activision.
So next COD on stage, Bungie and whatever they're doing next on stage.
Then at least 15 minutes will be dedicated to PSVR with new games announced, something similar to what happened in the Paris Games Week conference.
Fortunately after that I also expect many interesting updates to already announced games.
The Last Guardian with a final release date, new Horizon demo with release date, first Shenmue 3 official trailer, new trailer for FFVII remake, FFXV will be close to release date and will be pushed as well. Gran Turismo Sport will get a new trailer, coming in some form at the end of 2016. They also have many interesting japanese games to promote (Gravity Rush 2, Ni Oh, Persona 5, Ni No Kuni 2, new Hot Shot Golf) but E3 doesn't seem to be the right venue for those titles.
Big surprises will be God of War 4, Bend's horror game and maybe the new Sucker Punch game.
The only possible megatons I see are Crash is back, Kojima on stage to tease his new game announcing concept and genre, but it might be too soon. Overall I don't think it's possible to match what happened last year.
I also expect them to be working on a PS4 Slim revision (without upgraded components
) but again E3 might not be the appropriate event to announce such things.