It's been a very long time since I studied it, so you're likely right... but I thought that existentialism was the problem statement (life has no inherent meaning so we bring our own) and that absurdism and nihilism were both responses to it?
My understanding has always been that Absurdism says that while the search for meaning is absurd we should search anyhow if we feel like it (our happy friend Sisyphus). Whereas Nihilism's answer was more along the lines of Bukowski's "Don't Try." They both recognize that Sisyphus's job is pointless, but the absurdists suggest we keep pushing the boulder anyhow.
Am I missing something?
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.