I've seen more Unitarians, Universalists and Unitarian Universalists make comment -- positive, thoughtful or inquisitive comments -- about the doctrine of the Trinity in the last few days than I've seen in my twenty-five years as a (Unitarian) Universalist. (Parenthetical, because I do believe in the Trinity now, but it's not the sort of thing I lead with.)
So these references are more than today's observation of Trinity Sunday, but I'm at a loss to say what that cause is.
The least I can do is add in a strange and -- until the age of Google Books -- little seen South Carolina work. So little seen that I couldn't even get a copy when I was working on my (unfinished) thesis on Southern Universalism in 1992-3. But now you can read...
The Evangelists manual; or A guide to Trinitarian Universalists: Containing articles explanatory of the doctrines, tenets and faith of the Associates of the Primitive, Apostolic Church of the Trinitarian Universalists, in the city of Charleston. : To which is prefixed five introductory sections. And the eighteen articles of the Church, concluded with thirteen propositions, and an appeal to the Christian world. : With a copious index. (1829)
Historical note: Paul Dean was their minister for a few months. They dubbed him bishop, to the cocked eyebrows of the Universalists up north.