First, the early (march) 2016 is not a rumour, it is the confirmed Tesla plan.
Did that get confirmed? I saw rumors of it being shown off at an auto show, which isn't usual Tesla fashion, but hadn't heard anything official from Tesla. Link?
And I think TM3 will be much more developed by that time then any of the other cars was when they was first shown. Together with Teslas experience with the S and the X I don't think 2 years from prototype/alfa (remember: Tesla do not show concepts) to delivery is unthinkable.
Yes, Tesla has a "track record" for delays, but why does some people have to put "TM3 will not be ready for 2017!!!!" on repeat? Why are they talking about 2019/2020? TMS - Teslas first car designed form the bottom was "only" one year late if I remember correct?
Well, because of exactly what you said: Tesla is almost always late. It's just being realistic. The same argument about them having more experience could apply to the X as well. Heck, I remember in early 2015 Elon saying they could deliver the X soon, but wanted to do it right (for whatever that meant). Here we are entering September and it still seems they're readying things.
As for the S, If I remember correctly, the Model S was supposed to be 2010, but was soon pushed back. Elon is saying late 2017 which could mean December 2017 to early folks (founders) if lucky, but can easily slip 6+ months. Slip 6 months to mid 2018, and it's not hard to imagine them doing some early deliveries in the summer (to founders) and then a ramp up pushing most deliveries into 2019 if things don't go well.
Let's not forget the Model X is not only late, but they aren't likely to hit many volume deliveries in Q3 2015 either. Most folks will likely get an X in early 2016.