One ritual from the Roman Catholic tradition that expresses the belief in the holy trinity is the recitation of the Nicene Creed. Typically, the Nicene Creed is recited by the congregation at each Mass. It is recited after the priest has completed his homily.
This creed expresses some of the most central beliefs of the Catholic faith. One of the most important of these is the doctrine of the trinity and much of the Creed is devoted to that doctrine. When they recite the Creed, Catholics are affirming their belief in the trinity. Please follow the newadvent.org link to see the words of the Creed, which clearly describe Catholic beliefs about the trinity.