Are justice, equity, and compassion synonyms or are they something uniquely different? These terms get thrown together in the second principle and there is a pleasing meter to the triadic expression of "justice, equity, and compassion in human relations." Why not justice, equity, and compassion to all sentient beings and the whole world of which we are a part? I don't see any reason to stop with just human beings. What about environmental justice, and equity, and compassion - if we had a little more of that like Native Americans maybe we wouldn't be in this crisis of climate change having soiled our own nest, now, beyond recovery.
If we take the terms one at a time, let's start with justice, it has to do with the breaking of the law or a moral code of some sort or ethical principles. Of course, the problem is that some things are legal but immoral such as segregation in the Jim Crow days, or what we do now with our criminal justice system where we legally incarcerate so many more people of color than we do white people when they commit the same crimes, or abortion, which while legal, many people consider immoral. On the other hand, there are things that may be moral but which are considered illegal like nonviolent protest against injustice when demonstrators get arrested and "punished" for breaking the law advocating for some greater good.
Using Lawrence Kohlberg's model of moral development most of what we consider justice falls at the stage of "conventional" thinking and behavior. If a person moves to a post conventional awareness, he or she, might operate in a moral and loving way, but outside the law.
Most of the justice in the United States is retributive justice, instead of restorative justice, where the Code of Hammurabi, an eye for and eye and tooth for a tooth, is practiced. If you do the crime, you do the time. If you do something unjust you can expect to get punished and most of our society gloats and relishes the punishment even if it is with a guilty veil of decorum. Spanking a child for hitting a sibling doesn't work. Executing killers doesn't work either, but many would say that justice demands it and until this kind of justice is down, victims can't rest.
So what kind of justice is it that Unitarian Universalists are promoting and affirming in their human relations? I have never heard a good sermon on this nor read any UU theological treatises on it. If you have, leave a reference in the comments. It seems that this term "justice" is a more sophisticated concept which means a lot more than alluded to with the saying of the word, as it slips so shiny off the tongue, as part of the second principle.
Jesus and Buddha didn't seem to be big on justice. Jesus let the adulterous woman go who under the law should have been stoned to death. Buddha cautioned against vengeance and counseled detachment as a means of limiting or eliminating suffering. In A Course Of Miracles forgiveness is advocated as the means to dispel resentment and grievances. What is being suggested in the perennial philosophy is the rising above the ego compulsions for "getting even" as being another injustice. As Gandhi said, "An eye for an eye makes us both blind."
My Kind Of Church Music - Tom Dooley, The Kingston Trio