"Version" will matter more than "model year," just like your phone
Tesla isn't marketing the Model 3 based on the model year -- just the version number. Think about it like this: do you have an iPhone 6, or a 2015 iPhone 6? To the customer, this represents a psychological advantage, because it takes away the buying pressure during those times of the year when model changeovers typically take place. You don't really want a 2016 when you can get a 2017 in a couple months, do you?
As new versions roll out, Tesla will emphasize software updates and wireless maintenance with the Model 3, much as it does with the S. This enhances the perceived value for Model 3 owners, who won't have to worry as much about the depreciation cliffs that come with having an older "year" of the same car.