No one ever truly knows what exist and what doesn't. Just because they are unseen does not make them not real. Look at magic. Its only a select few that understand it. Many people have many different thoughts. So in reality who is to say that we even exist. Correct?
I'm not sure using the term Hell Hound is the best, but it seems to be used as an umbrella term referencing spectral dogs in folklore. I tend to treat them like spirits and associate them with the dead and with other certain kinds of spirits based on where the folklore is from. I'm actually rather fond of them.
When dealing with spirits from follklore, we take them with a grain of salt. Just like we don't consider Demigods from Hellenic lore as physical beings, we wouldn't treat Hell Hounds as physical beings, so I'm not sure that seeing one three times would actually kill you. If you work with Death, however, you can work with spirits under the umbrella term Hell Hound as a way of working with Death. If you treat them like spirits, treat them with respect.
I have to agree with Eissy. Hell hounds, I would say, is a poor term to use for the...creature you are referring to. It sounds like your talking about the mythical black dog that DeadPoet mentioned. Where did you hear this myth and why are you worried? Have you seen it three times? Less?
There is also another Myth, one a movie in America mentioned (though I can't remember what it was called) When your tiered, your eyes play tricks on you. It has happened to me many times, specially at night. I work at 3am, trust me when I say I have seen things that have made me slam on the breaks or swerve at times. Almost all cases it was my eyes. Truckers call this 'Devil Dog' or 'Demon Dog'. They see a black dog running at them while driving and jumping at the cab's windshield. When this happens, Truckers say they are very cautions because it usually means death is near. This is a myth though, derived from a few truckers who had hit and killed cars and people, saying they were trying to avoid the black dog in the road. Urban ledged really.
Myths and Legends, and fiction! They are beliefs, not truths. They have been written about in many books, such as Roald Dahl's "The Witches", and Conan Doyle's "The Hound of The Baskervilles".
There are many myths about black dogs, with many names. They are only real if you believe that they are real!
Many people talk of the light of the moon. Even "bathing" in moonlight. It is another myth. The moon doesn't have any light.
A hell hound is a real being. They are astral though. They are really demons in doggy form. Demons can do this as they can change their form. I think they are real as demons are real. This is my point of view hope this helps.
Black dogs guarding the dead don't kill you if you see them three times. As point of proof, I offer the fact that I'm here typing right now, very much not dead, having seen many, many entities that fall under the umbrella term for "hell hound"
