Complex sentence: Mars 2 and Mars 3 missions consisted of identical spacecraft, each with an orbiter and an attached lander, they were launched by Proton K heavy launch vehicle with a Blok D upper stage.
Split into simpler sentences: Mars 2 and Mars 3 missions consisted of identical spacecraft, each with an orbiter and an attached lander. They were launched by a Proton-K heavy launch vehicle with a Blok D upper stage.