There are a few places one can go for quick online answers: Wikipedia, the Stanford Encyclopedia of Philosophy and the Internet Encyclopedia of Philosophy. There are other places as well, but when these sources turn up in an online search they may be worth considering.
Let's just consider what Wikipedia has to say about these terms.
First, let's consider "existentialism":
Existentialism...is a tradition of philosophical inquiry associated mainly with certain 19th and 20th-century European philosophers who, despite profound doctrinal differences, shared the belief that philosophical thinking begins with the human subject—not merely the thinking subject, but the acting, feeling, living human individual.
What I pick out from here is that this is (1) this philosophy may no longer be very active, (2) it is concerned with subjective feelings rather than thinking, and (3) it is focused on the individual human being.
That article also discusses the confusion between existentialism and nihilism :
Although nihilism and existentialism are distinct philosophies, they are often confused with one another. A primary cause of confusion is that Friedrich Nietzsche is an important philosopher in both fields, but also the existentialist insistence on the inherent meaninglessness of the world. Existentialist philosophers often stress the importance of Angst as signifying the absolute lack of any objective ground for action, a move that is often reduced to a moral or an existential nihilism.
What might be worth noting here is (1) Nietzsche is considered both an existentialist and a nihilist and (2) nihilism involves the lack of an objective ground for action.
However, this suggests another term, "existential nihilism":
Existential nihilism is the philosophical theory that life has no intrinsic meaning or value. With respect to the universe, existential nihilism suggests that a single human or even the entire human species is insignificant, without purpose and unlikely to change in the totality of existence. According to the theory, each individual is an isolated being born into the universe, barred from knowing "why", yet compelled to invent meaning.
What this suggests is that maybe for some philosophers these two terms, existentialism and nihilism, may not be easy to distinguish.
Finally, consider "absurdism":
In philosophy, "the Absurd" refers to the conflict between the human tendency to seek inherent value and meaning in life and human inability to find any due to actual lack of any meaning or value. In this context absurd does not mean "logically impossible", but rather "humanly impossible".
What this suggests is the belief that there is no inherent value and so the search for it is in vain.
The question is:
Where do this ideas [existentialism, nihilism and absurdism] differ? Or are they all the same?
From the above, it may be possible to find differences in specific philosophers that would justify having three words for these positions, however, the labels given to those philosophers should be examined critically. Because of this one should look for evidence from the writings of these philosophers to justify any differences or similarities between them.
One might now consider a specific philosopher and ask how each of these three terms may or may not fit that philosopher's views.
Wikipedia, "Absurdism" https://en.wikipedia.org/wiki/Absurdism
Wikipedia, "Existentialism" https://en.wikipedia.org/wiki/Existentialism
Wikipedia, "Existential nihilism" https://en.wikipedia.org/wiki/Existential_nihilism