Many people say man is a trinity to give validity to something that does not exist: a God who is made up of three co-equal, co-eternal persons. Just how are we made in God's image? It could not be a physical thing since God is a spirit and humans are fleshly. Being created in the image and likeness of God, with God’s great attributes of love, wisdom, justice and power, the original man was much superior to the animals. Man is in a class apart from the animals in that he is able to reason, to plan for the future, and he has the capacity to worship God. Animals do not have intellect to reason with, but they live by instinct.
No matter what scriptures trinitarians grope for to prove their point, such as 1 Thess. 5:23, there is nothing in the Bible to support the belief that God is any more than one person. As Jesus said at John 17:3, his Father is the only true God. By calling his Father “the only true God” he shut himself out from being God or even a part or a Person of God. Otherwise, the Father would not be the “only true God.” The word “only” means, according to the dictionary, “alone in its class; without others of the same class or kind; sole; single; alone, by reason of superiority; pre-eminent; chief.” According to Jesus, his Father was, not only the “true God,” but also the “only” one. According to his own words, Jesus did not class himself with God.
"Tell me if Thessalonians, which mentions, body soul and spirit, can properly be used to support the belief that humans are a trinity."
The answer is no. However, trinitarians will twist any scripture to make it appear that humans are a trinity in some way. Desperation will make some people think and do crazy things.