How to check if three coordinates form a line Assume I have three coordinates from a world map in Longitude + Latitude. Is there a way to determine these three coordinates form a straight line?
What if I was using a system with bounds that defines the 2 corners (northeast - southwest) in Long/Lat?
The long & lat are expressed in decimal degrees.

I'll assume that by "line" you mean "great circle" -- that is, if you want to go from A to C via the shortest possible route, then keep going straight until you circle the globe and get back to A, you'll pass B on the way.
The best coordinates for the question to be in are cartesian -- the 3D vector from the center of the earth to the point on the map. (Lat/long are two thirds of a set of "spherical coordinates", and can be converted to cartesian coordinates given the radius of the earth as $r$, though for this question $r$ doesn't matter).
Once you have those points, which each one being a 3D vector), find the plane containing them, and check that it includes the center of the earth.
Or, as a shortcut, just mark the points on a Gnomonic projection map, and use a ruler to see if they form a straight line there. Unfortunately, a given gnomonic projection map can't include the whole world.
