I listened to the entire Code conference interview a few hours ago. My recollection is that Musk said there would be a significant Tesla "event" before the end of this year. He did not specifically say it was a "Model 3 event".
He said the topic of the even would be "the obvious".
I can see only one rational way to interpret that phrase: Elon will announce some high level of autonomous driving capability on the S and X. It will be similar to the way that AP was introduced in September 2014: suddenly all cars being built have new hardware, there is an event where Elon announces what the new hardware is for (in that case it was the first version of AP) and that the software will be released soon (in that case it took about a year, certainly longer than he intended it to take).
The event at the end of this year will, in my opinion, be an announcement of something like L4 autonomous driving, near the date of the event all S and X being built will have a new hardware suite of sensors, and the software will follow at some point in the future.
I'm as certain as I can be that is what will happen.