Define Composition of individual phase in phase diagram–three phase?

A binary system with three phases has only one degree of freedom and cannot be represented by an area on a two-dimensional phase diagram. Instead, there is a horizontal boundary line between areas, with a special point along the line at the junction of several areas. The compositions of the three phases are given by the positions of this point and the points at the two ends of the line. The position of the system point on this line does not uniquely specify the relative amounts in the three phases. The examples that follow show some of the simpler kinds of phase diagrams known for binary systems.