Hendiatris is a big word that means grouping things in threes. Do you know how sometimes you hear people say things like "stop, look, and listen" or "veni, vidi, vici?" Well, those are examples of hendiatris!
It's like when you try to remember something and you repeat it to yourself three times, like "I won't eat too much candy, I won't eat too much candy, I won't eat too much candy." Saying it three times helps you remember it better!
Using hendiatris sounds fancy and smart, but it's really just another way to help people remember things better.