Main reliability parameters of unrestorable unsymmetrical systems, ramified to level 3, with ageing output elements, are examined in this paper. A method of investigation of reliability parameters of ramified systems by means of generating functions is developed taking account of aging of the system’s output elements. Mathematical models of the probability distribution of count of output working elements, the duration of the system’s stay in each of its states and the duration of the system’s stay in the prescribed availability state are worked out in case when the lifetime of ageing output elements is circumscribed by the Rayleigh distribution.