Someone did the math above.

https://old.lemmy.world/comment/16391357

So, a typical pupil is around 2 mm in diameter in bright conditions. With the Rayleigh limit that results in an angular resolution of 1.22 * 60010^-9 m / 210^-3 m = 3.66*10^-4 rad

At a distance of 5 x 3 mi = 15 mi = 24.1 km this corresponds to a point to point distance of

tan(a/2) = (d/2)/l

d = tan(a/2) * l * 2 = tan(3.66*10^-4) * 24100 * 2 = 8.8 m

So in conclusion, with regular, human-like eyes he could discern points that are at least 8.8 m apart in the best case scenario. Discerning hair color from the color of the clothes would need a much higher resolution, and the horsemen are probably not 10 m apart from each other either. And again, this is a theoretical limit, real-world resolution would probably be significantly lower.