A model is presented for calculating the two-photon Doppler cooling of a three-level atom in a ladder configuration. This model is then applied to cool rubidium atoms, using the combined action of two lasers to excite the 5^(2)S(subscript 1/2) - 5^(2)P(subscript 3/2) - 5^(2)D(subscript 5/2) cascade transitions to an effective temperature of 300 nK, less than the single photon recoil temperature.