Abstract: In a triple cylinder plunger pump, each of the plungers is respectively actuated by one of three disc cams which are respectively connected to a common driving shaft and to a plunger. The three cams are identical in size and in contour and are positioned about the driving shaft with a phase angle of 120.degree. therebetween. Each disc cam is formed to have a contour consisting of two portions for urging each plunger to perform both a reciprocal discharge stroke and a suction stroke. The two portions are each respectively designed with a positive acceleration range, a constant speed range and a negative acceleration range. The system is so balanced that the pressure drop of any one plunger due to its negative acceleration motion can be compensated by positive acceleration motion of the next subsequently reciprocating plunger, whereby the discharge pressure of the pump is maintained substantially constant and pulsation in the discharge volume and in the suction volume is kept to a minimum.