As stated previously, originally, it was to be called the Model E. Ford blocked that from happening since they had previously applied for that trademark. Probably not worth Tesla's time fighting it, so they just change it. At that point, they could easily change it to a 3, which just happened to look exactly like the E using the font in Tesla, and still made sense, since it was the first generation 3 car (it was often referred to as "Gen 3" before we knew a name). Personally, I think they said "Screw Ford. We can get around it and still call it the Model E using our stylized E (the three bars), but on paper it's actually the Model 3." Then Adidas jumped in to sue, saying the 3 bars looks too close to their 3 bars... again, it's not worth Tesla's time or money fighting in court, even though they probably could have won that one, so it got changed to a number 3.