Complex sentence: Alongside Dmitri Shostakovich and Sergei Prokofiev, he is usually cited as one the three greatest composers of the Soviet period, who are sometimes collectively referred to as the three "titans" of Soviet music.
Split into simpler sentences: Alongside Dmitri Shostakovich and Sergei Prokofiev, he is usually cited as one the three greatest composers of the Soviet period. They are sometimes collectively referred to as the three "titans" of Soviet music.