Complex sentence: They released 3 SP from 1974-1975, on which appeared their hit single "Ulazak u Harem" (Entrance to the Harem) and they appeared as an opening act for the Deep Purple concert in Belgrade 1975.
Split into simpler sentences: They released 3 short play (SP) records from 1974-1975, on which appeared their hit single "Ulazak u Harem" (Entrance to the Harem). They also appeared as an opening act for the Deep Purple concert in Belgrade 1975.