Complex sentence: Secretariat hold the stakes record time for each of the three races, in addition his time of 2:24 for miles in the 1973 Belmont Stakes also set a world record that still stands.
Split into simpler sentences: Secretariat hold the stakes record time for each of the three races. His time of 2:24 for miles in the 1973 Belmont Stakes also set a world record that still stands.