Well, I don't think the NX will be revealed at E3. The trend nowadays is to do your own event during a time you don't have to share the stage. So, I expect either a Nintendo Direct or even a live event in March or April, with more details, and games at E3.
As for the question, I think it will be hard for Sony to compete. The problem with Virtual Reality is that the immersion factor which is the selling point is hard to show off. It's simply not fun to watch someone play, even with videos of the in game action. Without a better way to show this off, it's hard to see Sony blowing anyone away at E3. To be honest, Sony's never blown me away at E3. They seem to be, with some exceptions, always able to put on a solid show with good looking games, but I've never been dazzled.
Microsoft has done a good job showing Hololens off, and the tech is rather impressive. IMO this has more potential than VR does. The problem is, as it was with the Kinect, that Microsoft seems to think of good ideas in general that aren't good ideas for gaming. It's hard right now to see hololens as more than a novelty to be used for a handful of games, and a neat toy for techies, but I'd love to be proven wrong. The thing is, that MS does not have the strong developers of the other two to show the tech off. The best we got for Kinect was a Wii Sports knock off and a Sesame Street game, so I'm iffy on this.
So, basically, Nintendo has the ball in their court, but this doesn't mean they have an automatic win. Sony and MS can't beat them, but they can definitely beat themselves. We've seen a number of bad hardware reveals, such as XBone, PS3, Wii U, and a lot of ones that drew pretty mild reactions like the Wii and DS, which didn't blow gamers away, but later caught on with the mainstream.
In short, of course Nintendo can lose, cause the NX can suck. But even if they "lose" among the typical E3 audience, I don't know if that would really be a cause for concern. I think NX will skew far more casual than people think, so I'm less interested in the knee jerk reactions of gamers, and more keen on seeing how the media at large reacts.