I've been waiting for illuminated door sills ever since I got my car. I emailed the same company that make these door sills for the Model S and they said the Model 3 is wired differently so they couldn't make one for it.
I think they will all look a tad weird seeing how the model 3 sill plate is just taped/glued onto bare metal. No wiring paths, and raised a decent height. Though I agree, I wish it had it flushed like the model S.