The paper allocates a stochastic model on threesimilar units three-phased mission system. The developed system
consists of units working in parallel, series and parallel
configurations respectively. Initially, the three similar units are
operational. Each component has only three states: good,
degraded and failed. In this case, the single repair facility that
repairs the units in first come first serve (FCFS)pattern has been
thought of. Using Semi-Markov Process and regenerative point
techniques, various measures of the system performance at each
phase are obtained. The system has been analyzed graphically
taking a particular case. Various conclusions are made regarding
the reliability and cost consideration of the system at each phase
as well as for the whole system(as combined Phase I, Phase II,
Phase III).