No, 3-legged tables are perfectly stable on the floor too, even better than usual 4-legged ones (which allows you considerable saving on screws) - except when some exoteric Incarnation gives them a good shake, of course
Coanda, are we sure we're talking about the same software? I just tried it bluntly one minute ago: a randomly drawn extruded
surface (mind you, not
line) subtracted from another randomly extruded surface. Well, apart from one degenerated surface (easily removed with an "optimize surfaces") the outcome was perfectly OK. The disturbing side is my vague feeling that when I'll try the same tonight at home it just won't work.
I concluded that AC3D must be of the female persuasion (Lisa, don't growl that loud now!): there's always something to it that we poor defective XY models just cannot understand...