Fully developed liquid-metal flow in a system of three straight rectangular ducts is investigated. The ducts are electrically coupled by common conducting walls covered with an imperfect insulating layer. A numerical model of magnetohydrodynamic (MHD) flow in the system is described. Since no additional assumptions, such as in the core-flow solution, have been made, this model can be used for the analysis of MHD flow in parallel ducts with nearly perfect insulating coating. Any orientation of the applied uniform magnetic field is possible. Electrical conductivities of the dividing and exterior walls, and of the insulating layers in individual channels can be varied independently, as well as characteristics of insulation imperfections in each channel. A restriction of equal pressure gradients in all ducts is imposed, and the flow partitioning between parallel channels is examined. Results of the numerical simulation of the influence of insulation imperfections on flow distribution and velocity profiles are presented.