My own understanding is: all three of existentialism, nihilism and absurdism are responses to the same question (that of the meaning of life). All three agree on the fact that there is no inherent, "natural" meaning.

Nihilism gives you nothing more. Absurdism claims you must embrace this lack of meaning and thrive in spite of it. Existentialism claims you can create your own.

Camus thought Existentialism was "cheating" and trying to deny the correct conclusion that life is meaningless. Also, he diskliked Sartre a lot.