。
The third installment was a whole level above.
Literal
3rd-work [topic-は] level [subject-が] one-step above was.
A brief appraisal. 作 is the counter for creative works (movies, games, books, seasons), so 3作目 ('third work') refers to the third installment in a series. 一段上 is a fixed spatial-metaphor phrase: 一段 ('one step/stage') + 上 ('above, higher'), meaning 'one level higher.' A compact but specific way to express 'better than the previous one.' Natural review speech.