Plane. Three points lie in a unique plane. Two points can always be joined by a straight line (in Euclidean space) and that line can be part of a plane. Think of it as a hinge and add a third point not on the same line as the other two. Swing the plane until it captures the third point. Now you have three coplanar points, and their common plane is the only one that contain all three points. A fourth point will not necessarily lie on the same plane as the other three.