When checking the accuracy of a flat surface against a datum surface, there will theoretically be more than three points on the flat surface touching the datum surface (gage).

Datum Surface

A reference surface or point from which measurements are taken to ensure parts are manufactured, or assembled correctly.

Flat Surface

A surface that has a continuous planar extension in which any two points can be joined by a straight line that lies entirely in the surface.

Gage

A measurement or inspection tool used to verify the conformity of parts to specified tolerances or dimensions.