The objective of this work is to study distributed approach for solving three-phase optimal power flow (OPF) problem in unbalanced power distribution networks. A full three-phase representation of distribution networks is considered to account for the highly unbalanced conditions. All distribution network's series/shunt components, and load types/combinations are considered. The OPF problem has been successfully implemented and solved in a centralized and distributed approaches, where the objective is to minimize the active power losses on the feeder. Case studies are carried out using the IEEE-37 Node Test Feeder.