Look carefully at the “Tracking Algorithm” section on pages 7 and 8.
ΔL, ΔR, and ΔS are the changes in rotation of each tracking wheel, where ΔL and ΔR are from the left and right (parallel) wheels.
The only time ΔL is used is in step #5 to calculate the new absolute orientation. (It’s also used in step #7, but that is only to check if the robot has not changed rotation by checking if ΔL == ΔR.) For the rest of the calculations, the document says only to use ΔR to measure how far the robot moved in the forward/backward direction. It could just as well use ΔL, but then it would need to account for the fact that the tracking wheel is in a different place on the robot in step #8 by subtracting s_L instead of adding s_R (if I’m remembering this correctly).
Basically, the algorithm doesn’t really need 3 tracking wheels, it needs 1 to measure forward/backward movement and 1 to measure left/right movement, and something to measure angle change. Because this algorithm was developed before the VEX Inertial Sensor came out, the Pilons opted to measure angle with two parallel wheels instead of the gyroscope. Now, however, the inertial sensor is a good replacement for the parallel wheels.