Trinitarian doctrine declares the following three things:
1. There is only one God.
2. The Father is God, the Son is God, and the Holy Spirit is God.
3. These three persons are eternally distinct.
Monotheism, according to the Mirriam-Webster dictionary, is defined as "the doctrine or belief that there is but one god". As can be seen from the first plank of Trinitarian doctrine listed above, Trinitarians insist upon the fact that there is only one God, making Trinitarianism a true form of monotheism in every sense of the words.
Unitarianism, again according to the Mirriam-Webster dictionary, is the belief that "deity exists only in one person". Trinitarians are not Unitarians.
Unitarians include Muslims, Jehovah's Witnesses, Arians, and Socinians. Trinitarians consider Unitarian doctrine to be heresy -- a doctrine contrary to the truth.
The Trinity Scripturally Supported