My reaction to the Trinity has varied over the years. I rejected it as a teen and in university, probably my first step on the road to UU'ism (being as the first U is Unitarian). Then I looked at it as more of a mythological idea, representing God as working in the world in different ways. Heretical and technically incorrect, or so I am told, but I was not approaching this as theology but how it might be seen as mythology and as a literary image/device.
For a more literal understanding, and one that kind of makes sense in the 21st century, might I suggest borrowing superposition from quantum mechanics? God has three states and exists in all them until observed. We cannot know God entirely because when we "measure" God in one of those states, God becomes that state and the others temporarily disappear. To measure God as all three is simply impossible, which is part of what makes God transcendent. "Schrodinger's God" if you like. Probably also heretical and incorrect in some profound way, but at least sensible to a post-modern quantum reality. However, it also makes some of the theological gymnastics around the concept look positively simple to understand.
However, as to how to mark it in church? Perhaps skip the sermon and instead do a service of lessons and short commentaries, each lesson presenting an image or story related to one of the Three (Parent, Child, Spirit, or whatever you call them in modern, non-sexist, non-patriarchal language). I have done similar UU services using poems and readings on a theme (e.g. Love in one case) for the "lessons".
gordsponderings.blogspot.com
