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Mathematically, three points determine a plane. In other words, for any three points chosen on an uneven floor, there is an even floor with some slight tilt that passes through these points. A stool with legs touching the floor at these three points doesn't know that it is not sitting on this even floor.
When a four-legged stool, on the other hand, is to be set down so that its legs hit the floor at a certain four points, these four points are not necessarily contained in a single plane. Assuming the stool is designed for an even floor, it may be able to touch with only three legs at once, wobbling between different choices of three legs.
citation: David Greenspan, Lexington, KY USA