just re read that and it was one of my scramble sentences
the angles that the three legs are organised are done so usually in a way so that they are independant of other legs, in a four legged item, such as a standard chari or table, or a flat based chair or table, much of the base is reliant on other parts of the base, think of how you cant keep that finger before your pinky up without the other fingers, thats the case of a four legged or flat based table or chair, when they have three legs, its like three thumbs.