The prototype for this icon was the mysterious appearance of the Holy Trinity in the form of three travellers to Abraham and Sarah under the oak of Mamre. This particular icon by Andrei Rublev does the best at most fully expressing the dogma of the Holy Trinity: the three angels are depicted in equal dignity, symbolizing the tri-unity and equality of all three Persons.
The Father, Son and Holy Spirit are one God and three Persons. It was Jesus who revealed this to us (He is God the Son). It is the Trinity that makes Christianity truly distinct. Come to Mass this Sunday and learn more.