do they exist ?

creepypasta's / yokai's/ 3am creatures do they exist ? ynm? that is a mystery for everyone ..... no one really knows the answer to that .... the paranormal / universe / etc is a fickled thing full of mysteries !?!?!

Personally, I would say that some spiritual beings are more likely to exist than others. Creepypastas, are made up stories, they may be based on pre-existing folklore or spiritual beings, but the creatures within the stories are typically made up solely for the purpose of the story, often with fake folklore or surrounding mythology. Same goes for 3am monsters/creatures, they are usually created for the purpose of scaring people, tapping into primal fears, though they may be based on something much more long standing, they themselves are often named and created for the story alone, with any surrounding mythology being created to scare people.

Yokai are slightly different as these are pre-existing creatures and spirits from Japanese folklore, they are often more filled out, with a deeper cultural basis than the others. Being found integrated into all apsects of culture and life, not solely for the purpose of fear but also for positive outcomes as well.
Though of course, as with all spiritual or supernatural beings, everyone will have varying opinions on their reality or lack of.

This thread has been moved to Spiritual Creatures from Welcome.

Creepypastas are fictional though some may be inspired by actually spirits.

Yokai are real. They are elemental spirits from my limited understanding of them.

Never heard of 3am creatures before...

Whether or not they exist outright, is one thing, but existing at all is another. I'll focus primarily on creepy pastas as I'm unfamiliar with the other two things you've mentioned. I'll use Slenderman as an example here. Did Slenderman exist, prior to the story being told? No. Did he exist as it was being written? Only as a thought in the mind of his creator. Did Slenderman come into being as people read about him? Again, only as a thought, though now a collective one, rather than an individual one. So does Slenderman exist? Well yes and no. As a physical being, Slenderman existing is entirely out of the question. As a spiritual being his existence was inevitable given his popularity.

To explain, he started as a thought, as we've established. As young, impressionable readers began to believe in him, he became something more like a thought-form. And as stories of this thought-form spread and circulated, he gained a correlative identity. He evolved from a thought-form into what is known as an Egregore. A local spirit (a spirit that is an extension of yourself) shared non-locally via collective unconscious. In this state, the identity of the spirit is subject to change as every new story about it takes hold of the minds of its readers. Far more powerful than a thought-form, and beyond the control of an individual.