Complex sentence: In January 1919 "Collingwood" was transferred to Devonport where she was assigned to the Reserve Fleet, which was redesignated as the Third Fleet on 18 March on the dissolution of the Grand Fleet and she became its flagship.
Split into simpler sentences: In January 1919, "Collingwood" was transferred to Devonport and assigned to the Reserve Fleet. Upon the dissolution of the Grand Fleet on 18 March, the Reserve Fleet was redesignated the Third Fleet and "Collingwood" became its flagship.