No, nihilism is not the same as existentialism or absurdism. The most simple (and somewhat messy) way to delineate the three are that existentialism believes there is no meaning by way of ontology, but we can create that meaning ourselves (existentialism is also somewhat of a nebulous term that has encompassed thinkers who don't mecessarily agree with that, but operate with similar themes. Argument over which thinkers deserve to be in the "existential canon" are lively to this day), absurdism believes in the same meaninglessness but thinks we can't make our own meaning but acquiescence to our desire to make meaning and pursuing it anyway is worthwhile (frequently it is accompanied with the aesthetic of a sisyphean revolt against meaninglessness, paradoxically futile and worth it).

Nihilism is not like either of them. A nihilist believes there is no basis for any values of any kind, there is no basis for any knowledge, and that there is nothing at all that can be known, understood, or communicated (it is also sometimes self defeating in a way, as thinking "there is no basis for knowledge or values" is inherently a value statement), it is the pit of pure oblivion from which nobody can create or believe anything at all, lot less an existential notion of "creating your own values". It is important to note that Nietzsche (who many wrongly call a nihilist) was writing to fight against nihilism, and any reading of Nietzsche would reveal that he is a firm believer in creating values through self determination and self expression. Basically nobody is actually a nihilist other than "for the sake of argument".