In this paper authors present the results of their studies on the selection of fuel dose in three-part injection system Common Rail of marine medium-speed combustion engine. The study was conducted on a special test stand, which allows the use of different pressure in the hydraulic tank and various times of injection with doses divided into three parts. Also there has been performed the measurement of single fuel dose mass. The plan of research also included the three values of the two input values (pressure in the hydraulic tank and the total time of injection) and one output - the total mass of the fuel dose. Based on the statistical analysis of measurement results there have been determined the forms of functions approximating the tested relations and have been presented the analysis of the accuracy and their measurements match. Presented polynomials of accuracy (function of researched object) allow a reasonable selection of the basic parameters of dosing for the three-part fuel injection.