Complex sentence: He won at least 3 championships before June 1909, when the "yusho" system was established by the "Mainichi Shimbun" newspaper (the Japan Sumo Association officially recognised the system in 1926).
Split into simpler sentences: He won at least 3 championships before June 1909. The "yusho" system was established by a newspaper in June 1909 (Sumo Association admitted the system in 1926).