As always, I could be wrong, and my statement even suggested that I was unsure. That being said, I'm going to look tonight and see if I can locate such a quote about what numeral / icon / character would be used, according to Elon.
As a side note, why would anyone interpret "Perhaps three horizontal lines" as "---" in this situation (I'm not saying that you're wrong about the possibility, I am asking why any car maker would make a car named "Model ---"?) "Model ≡" actually makes sense. "Model ≡" also fits with the, "it was supposed to be 'Model E'" story.
As for the use of the "≡" character vs the "3" numeral here in the forum, why would we think that our usage in this forum would be any basis for the actual outcome?
And sure, he does use "Model 3" in his tweets... it's a single character to press on his keyboard, very simple, easily searchable, and public friendly. On the Mac, I must enable Unicode Hex Input, then type "Option+2261" to get a "≡" Though, I've setup Text Shortcuts so that when I type "M3" my Mac changes it to "Model ≡" automatically. Not a lot of people will know these tools / features exist, or how to use them.