As I read the first part of To Ablabius, On Not Three Gods by Gregory of Nyssa I cannot help but pause because it sounds so Tritheistic! I may be misunderstanding what he is trying to say but it seems that his introductory argument follows as such:
(1) Peter, James, and John are three individuals.
(2) Peter, James, and John are all men.
(3) We can speak of Peter, James, and John as being three persons sharing one substance, namely “humanity”
(4) The error of language is speaking of men as “many men” rather than “humanity”.
(4) The error of language is speaking of the Trinity as “Gods” rather than “God”.
(3) We can speak of the Father, Son, and Holy Spirit are being three person sharing one substance, namely God/Divinity.
(2) The Father, Son, and Spirit are all God.
(1) The Father, Son, and Spirit are all individuals.