Yeah, I"m not convinced waiting is going to financially work out the same as a normal car: Let's say you buy a base 35k TM3 new.
1) Your actual price will be $27,500 (plus tax and minus any state rebates you may have.
2) After one year the average car depreciates 20% of so which is $28,000
3) I would say, because the Model 3 will likely still be constrained for well over a year after production starts the depreciation will be less, say 15%
4) At the end of year two, let's say Tesla is finally meeting demand, more or less, car still more advanced than most out there and is holding it's value a little better than average, has deprecated 25% now $26,250
5) Year 3, the time a lot of folks traditionally trade in a car because most manufacture warranties and leases expire. Tesla still under warranty holds a little better than average still, let's call depreciation at 35% $22,750
So really you'll have to wait until year 3 to buy a Tesla for less than you would drive it off the lot brand new. What advantage would you have on waiting until 2021 to buy the same car you can have in 2018?