Simple and plain.

In my opinion,
Existentialism - Questioning of ones individual purpose and meaning.
Nihilism - No matter what is done it will be meaningless in the grand scheme of things as we are less than a speck of dust in the universe.
Absurdism - We are slaves to the ever changing world around us, and are heavily dependent on the mercy of the things that we need to survive.

en.m.wikipedia.org/wiki/Absurdism

Peep the "Relationship to existentialism and nihilism" section and it does a pretty good job explaining the differences.

PS Camus is goated