"Dichotomy" but for three
Trichotomy is listed in Merriam-Webster. In mathematics one also uses this term when referring to the fact that a real number is either zero, positive or negative.
I think it's sufficiently clear and correct. Lots of dictionaries list trichotomy with the definition "division into three parts, elements, or classes".
Depending on the context tripartite ("in three parts") might be appropriate. It's certainly more commonly used than trichotomy in my experience.