Of course! A 3-legged table wouldn’t wobble! Why? Any three points define a plane. Why did I never think of that?
So you’ve purchased your coffee and chosen to sit at one of those round outdoor tables. As you lean on the table to write comments on a paper, it rocks annoyingly, possibly spilling some of your coffee. You try moving the table slightly on the uneven pavement, hoping to stumble into a stable configuration for its four feet, but several attempts fail. Eventually you resort to shimming one of the table feet with a piece of folded up paper, or a stack of sweetener packets, and this creates at least a metastable condition. Looking around, you notice that many other tables have similar combat repairs, so that the cafe looks like a furniture trauma ward.
Why don’t these tables have three legs instead of four? With three legs, they wouldn’t rock on uneven surfaces because any three points define a plane. You wouldn’t need those adjustable table feet that no one ever bothers to adjust because it’s so awkward to lean down and twist them. While each leg would have to be slightly bigger, you’d have fewer assembly or machining steps to perform. Is a 60 degree angle that hard to produce in this day and age?
Steven Postrel, Design Puzzles