Maybe it's just smoke and mirrors, but Shuhei Yoshida claimed that there were more games they could have revealed at E3 but didn't:

‘’We have no plans to talk about [Sic], but there are things we have held – we chose not to show during this E3 –  of course there will be some more news coming out of our teams later this year.’’

-Shuhei Yoshida, President of Sony Worldwide Studios

http://www.psu.com/news/33251/sony-held-back-e3-2017-announcements-admits-shuhei-yoshida

Take that however you will, but I must admit that the quantity of games announced during their E3 presentation seemed rather lacking.  Hopefully, we'll hear something soon during the last half of 2017.

So aren't Sony the only ones who actually have 3 conferences a year? The others have 1 right?

Saveing some for other parts of the year...... could be a mistake by them?

Based on last year Sony also only has 2. Last year they skipped both Gamescom and PGW afaik.

Figured this was common knowledge. I think they held back more this year than normal because they didn't think Nintendo would step their game up like they did.

But they had TGS

Sure, but internationally that isn't nearly as big as E3 and PSX and it's extremely heavily focused on the Japanese and Asian markets.

E3+TGS+PSX=3...

Obviously they kept games aside, they do it everytime to have things to talk about all year long. That way, we have several "Sony conference live" with the exclusive focus of all websites on them. It has never been a mistake so far, don't know why it would be now. 

Microsoft didn't do a Gamescom conference last year, not sure about 2017...

EDIT: The TGS is a huge event. It's just the main event for the companies of one of the 2 main countries in the video games industry...

Nintendo have their Directs once in a while. 

Most of the games at the April Direct weren't shown at e3. 

MS only did E3 last year.