Complex sentence: Gaia also mated with Tartarus to produce Typhon, whom Echidna married producing as their first three offspring Orthus, Cerberus, and the Hydra.
Split into simpler sentences: Gaia also mated with Tartarus to produce Typhon, whom Echidna married producing several monstrous descendents. The first three offspring Orthus, Cerberus, and the Hydra.