Back in 2010, Tesla Motors put out a promotional video titled "Work At Tesla." In that video (embedded below), at the 1-minute 23-second mark, a clay model is shown (see image above).
Now that we know what the actual Tesla Model 3 looks like, it's believed that the clay model in the promo video was actually an early design of the 3. If you look closely and focus mostly on the front and rear overhangs, you'll notice that the clay model is much closer proportionally to the Model 3 than it is to the S.
Actual Tesla Model 3
Tesla Model S For Comparative Purposes
If the clay model is actually what later became the Model 3, then we got our first look at it some 6 years ago, which sort of does make sense since the typical design/development cycle often takes ~5 years in the automotive industry.
(And we also learned with the 3 (and this earlier clay mock-up) that Tesla cars have always basically had the same efficiency-driven shape from concept through production)
Source: Tesla Updates, Hat tip to Teng Y!