How do I say "every three hours" in one word?

This kind of questions can have two possible answers.

1. We half-resurrect, half-invent an obscure half-Greek, half-Latin word that nobody ever uses or understands. Then as soon as you need to say "every 48 hours", or "every 113 weeks", you have to ask the exact same question all over again, because you have no idea what the Latin for "48" is, or the Greek for "week".

2. We encourage you to not reinvent the wheel and just go with "every three hours". You used these exact words to explain the concept to us, so there's nothing stopping you from using these exact words to explain the concept to others. The construction is perfectly natural, ubiquitous, universally understood and extremely productive to boot. You can just say "every" followed by any number followed by any unit, and you're done. Doesn't have to be time units, even. Can be miles, liters, degrees, Joules, leaves, houses or ticks. Which is sort of the whole point of languages: not to come up with a dedicated word for absolutely everything, but to have a bunch of very simple words that can be combined in very simple ways to form very complex thoughts.

To me, it is a no-brainer that answer (2) is vastly superior. And so it's also the one I'm going with here.

I have seen this:

"trihoral"

adj. Occurring once in every three hours.

https://www.wordnik.com/words/trihoral

http://machaut.uchicago.edu/?resource=Webster%27s&word=trihoral&use1913=on

http://www.thefreedictionary.com/Trihoral

but I leave the word to native speakers on the matter.