The Holy Trinity, Three-in-One is a theological doctrine, according to which the Father, the Son and the Holy Ghost were united in Christ. On the whole the Trinity is considered to be a mystery, which is impossible to understand by human minds.
See: Robert Campin Mourning Trinity (Throne of God).
El Greco The Holy Trinity.
Masaccio The Holy Trinity with the Virgin, St. John and Two Donors.
Andrei Rublev. The Old Testament Trinity.
Theophanes the Greek. The Trinity.