The one that represents Jesus is touched by a circle that stands for his human nature, whereas the corner indicates his divine nature. What he did in one he did not in the other. St. Augustine compared the Trinity with love that involves a lover, the loved one and a spirit of love beteen them. It may also be valuable to see the one universe as made of space, matter and time. Time by itself consists of past, present and future.
Fire generates heat and light.
Thus fire, with its light and heat is one thing. Multiplicity in unity is a very common phenomena. Further, some have pointed to the fact that Muhammad was simultaneously a prophet, a husband, and a leader. Self-knowledge demands a distinction, a multiplicity, between knower and known. This brings us to another analogy for the truth of the Trinity, that of man's mind.