Switching Functions of Three Variables

A switching function is a function of variables which take only the values 0 and 1, and which takes only these values itself. There are 256 different switching functions of three variables, but only 218 of these really depend on all three variables. A switching function of three variables can be expressed in terms of switching functions of two variables. For example F1{F2[A, F3(B, C)], F4(B, C)} can be shown to represent any function of A, B, and C if F1 F2 F3 and F4 are suitably chosen switching functions. The problem solved is: for each switching function of three variables, what is the least number of switching functions of two variables required express it?