Three objects with masses m1, m2, and m3 and specific heat capacities c1, c2, and c3 have initial temperatures T1, T2, and T3, respectively. Objects 1 and 2 are first placed together in an insulated container and reach an equilibrium temperature T12. Without any energy exchange with the environment, the combined system (objects 1 + 2) is then placed in thermal contact with object 3, and the entire system reaches a final equilibrium temperature Tf. Derive an expression for Tf in terms of the given masses, specific heat capacities, and initial temperatures.