Ternion is a rarely used word that can be challenging to find synonyms for. However, there are a few options that could be used to replace ternion. One option is "trio," which means a group of three. Another possible synonym is "triad," which refers to a set of three closely related things. "Tern" is another potential choice, which simply means a group of three. Additionally, "threesome" is a more informal term that could also be used instead of ternion. While these alternatives may not be as precise or unique as ternion, they can serve as suitable replacements in certain contexts.