I don't think that Jon's reply is correct. There is no a priori
reason to assume that when you have three or more point charges (or masses
in the case of gravitational forces) that superposition must hold. It is
always possible that you could have many-body contributions to the force
that depend on the new coordinates (as you do in solid matter for
example). The argument for superposition then must be based on
experimental results, i.e. that happens to be an inherent feature of the
electrostatic force.
Dr. Mark H. Shapiro
Physics Department
California State University, Fullerton
P.O. Box 6866
Fullerton, California 92834-6866