This paper presents a complete analysis of a three dimensional retro-PN guidance law, suited for intercepting targets that are of higher speeds than the interceptors. It proposes a somewhat counter-intuitive guidance law that uses a negative navigation constant (as against the usual positive one) to show that this modification makes it possible to achieve collision conditions that were inaccessible to the standard law. An analysis for three dimensional engagements is presented in a modified polar coordinate system and complete capturability results are obtained. Simulation results are given to support the theoretical findings.