This paper is devoted to the study of the Reissner-Nordstrøm-de Sitter black holes and their maximal analytic extensions. We study some of their properties that lays the groundwork for obtaining (in separate papers) decay results [17] and constructing conformal scattering theories for test fields on such spacetimes [16]. Here, we find the necessary and sufficient conditions on the parameters of the Reissner-Nordstrøm-de Sitter metric -namely, the mass , the charge, and the cosmological constant-to have three horizons. Under this conditions, we prove that there is only one photon sphere and we locate it. We then give a detailed construction of the maximal analytic extension of the Reissner-Nordstrøm-de Sitter manifold in the case of three horizons.