Encode your message as a number that will then become, say, the x-coordinate of a point P in three-dimensional space. Choose two other coordinates randomly. Now select five planes that all intersect at point P and assign each courier to a different plane-defining it for the courier using the coordinates of three other (non-P) points in the plane.
Two nonparallel planes meet at a line, and any plane not containing that line but intersecting it will hit the line at a single point (see figure 1). So, knowing the planes of any two couriers would give the enemy no information about point P, but any three couriers together could find the critical point readily and determine the x-coordinate to uncover the full message.