I guess the Sun would have to be a lot closer in a flat Earth model otherwise what would there be to stop the Sun illuminating the whole Earth at once? The Sun emits light uniformly over the whole of its surface area so if it was shining onto a flat surface then it would light up the whole of that flat surface leaving no night.
Even at 3,000 miles it would be shining over the whole earth. What would stop it?
When I was new here I thought they believed that the sun was a literal spotlight and I asked a few times how that worked and got no response.
Actually, they don't believe that. Mostly they believe that the sun does shine light in all directions. In the real world that would mean it shines over the whole plane so we'd be in perpetual daytime.
Their fudges to get around that are some weird version of perspective where things disappear behind merging perspective lines or EA which bends light upwards. The second of these would sort of work. I'm pretty sure there's no actual evidence of this effect even existing but if it did then it's a much better explanation than "perspective".
If the sun were as close as 3,000 miles then you'd be able to measure the angle to it from a few places known distances apart and use the different measurements to triangulate and show the distance.
I have suggested this be done multiple to no response although I did once get a reply from someone (I forget who), they said that he had seen that done and would find the results and post them. He never did.