[philman_36]

Tell me...what is the curvature at 3 miles?
Still waiting.

[thepoodlebites]

Tell me...what is the curvature at 3 miles? Still waiting.
—
Why are you arguing about the curvature of the Earth when you believe that the Earth is flat? Your estimate using a parabola is approximate for three miles but quickly loses accuracy for longer distances.
For celestial mechanics, trying to fit a parabola to a circle does not provide the precision for scientific calculations.
You can find several sources for the answer to your question. I looked up one source for you. [1]

1. https://www.quora.com/When-NASA-tells-us-that-the-Earths-curvature-is-approximately-8-inches-per-mile-squared-what-is-the-true-and-exact-value