When you are adding more pump sections you have to take in to account for the torque capability of the input drive shaft and spline coupling that goes into either sections. Your maximum PD for a 7/8″ drive shaft is 53. Also for a coupling is 53.

The “P” is for the psi, which is the relief valve setting of each section on it’s own. The “D” is for displacement in the amount of gallon per revolution of each section. If you run all three section at the same time you will need to think about the pressure at the same time and the coupling that goes between the front & center sections. All sections need to be able to give same amount of torques to each section for the operation to run smooth.