Look back at the post that started this, tzadik. I believe I said ‘if the Earth was a perfect sphere’. Not to slight spheres who aren’t completely round, but they are not perfect. 🙁
So yes, it is straight on the surface.
Saying it is “straight on the surface” is meaningless, because you have qualified the term “straight” with “on the surface.” So it is only as straight as the surface is.
And a triangle as three straight lines.