Unitarian Universalists believe that each person is free to search for their own personal truth on issues, such as the existence, nature, and meaning of life, deities, creation, and afterlife. UUs can come from any religious background, and hold beliefs and adhere to morals from a variety of groups or religions. I do not think that not believing in the Trinity applies here as much. It may apply to a particular Christian Denomination, because as you can see Unitarians consist of all religions and beliefs.