I too think that this will happen--but only for low-volume deliveries.
I don't think Tesla will stop the whole "beta testing" car thing that they did for the S and X as they ramped up. I think the Model 3 will still start off at a relatively small trickle.
I believe that, however, the late 2017 / early 2018 mark has always been an internal mark for high volume production.
Here's what evidence and speculation I think may contribute to this situation:
1) The Gigafactory is going to have full car battery and cell production capabilities by late 2016. I think it makes sense for the Gigafactory to produce the completely new battery tech (the larger cells they're using) from the get-go, rather than have the Gigafactory tooled and designed to produce gen1 S and X packs.
2) Mules are, essentially, already on the road. MX mules appeared six months before production, approximately. Applying the same timeline would result in late 2016 for M3 production to begin.
3) It's been known that they've been working on this for a long time.
4) Car definitely looks like it was designed for ease of production (people say the all glass is roof, I say it's probably much simpler to place glass on a chassis like that given their expertise with the MX windshield.
5) Elon already is teasing about the final designs of the car.
6) Engineers stating this is a "production prototype" on the test drives.
7) There are no unproven challenges in this car. Everything it has, has been done on the S and X already.
8) SpaceX and Tesla employees taking first deliveries--they can have a more true "beta test" than the S and X ever could have.
9) The west coast to east coast priority, and stores taking reservations first--they want the early cars to go to places near heavy, existing infrastructure first. Near stores, near high-volume Supercharger corridors like in California, in states that allow service centers. Or in other words, they'll be delivering cars but still needing time to build infrastructure. Having Model 3 development costs largely finished will free up tons of funds for infrastructure.
10) Pre-orders dictating priority for infrastructure.
11) Anti-Tesla laws won't be going away until Tesla is a mass market car, and not a rich man's toy. The earlier Model 3 deliveries start, the earlier these laws can be repealed for infrastructure in these states.
12) More revenue without raising more money through Wall Street--both for timing of development CapEx and for earning sales, it'll be very hard to create more factories and gigafactories in time if they cannot start until late 2017 / early 2018.