Source: TRB. Highway Capacity Manual. 2003

A, B, and C are the approach volumes on each leg.

              X = (A + B + C) / 2

Where:

 X - C = A to B and B to A

X - B = A to C and C to A

X - A = B to C and C to B

Figure 13.1. Directional Counts at Three-Leg Intersections