I did it to this house https://properties.mikescnr.com/property/view/749
You can already see it says 3 garages on there (but not in-game yet).
I also did it for another one but I forget what/where it was.
I don't think the ones you posted are wide enough for 3 spots to be honest.
I think the garage doors can be imagined as divided into sections by the square texture styles on them, I used that as reference to park vehicles in front of them to keep it simple. Based on that, I was guessing 4 spots for the 749 house you showed to now have 3 slots; then 3 for the other two houses I linked, since I was able to park cars their lol
EDIT: I think the way you did it is fine, I think you are right now that I see how it will look, only about two needed that adjustment, the 3 garage one you did in SF, and the 4 garages on you did in LS.