[hydrogen7800]

This sounds like the principle of "designing for exact constraint". You learn about under-constrained and over-constrained typically in a strength and materials course in ME undergrad. An simple case of exactly constrained is a 2D beam with a pin/hinge at one end, and a roller support at the other. The pin removes the 2 translational DOF's and the roller removes the third rotational DOF. A beam with a pin/hinge at both ends is over-constrained and is "statically indeterminate" (or over-constrained), which requires you to use other techniques to find the stresses in that beam, such as principle of "virtual work", or some others I've forgotten. For more complex structures, there is a formula which has as its inputs the number of members, number of joints, type of joint, etc, and will give an integer output which will tell you over, exact, or under constrained. Although a 3 legged table will never wobble, they will more easily tip, and the surface of 4 legged table can be considered somewhat flexible, and provide an additional DOF that keeps the legs in good contact with an irregular floor.