In recent years, the exploration for other planets are really popular among many researchers. It seems that Rover is the one of the best application to know the details of the surface of the planets. For example, Spirits and Opportunity; the rovers of NASA, still keep exploring on mars, and we can get a great number of information of Mars from those rovers. Therefore, such a kind of rover is very important and effective for planetary exploration. This paper proposes the concept of the horizontal polyarticular expandable 3-wheeled planetary rover, "Tri-StarIII" which based on the expandable rover. The development of the arm, wheel configuration are presented in detail.