Ecumenical Venerables of Asia Minor who lived in the 4th century: St. John Chrysostom, St. Basil the Great and St. Gregory the Theologian. They are called the Three Ecumenical Venerables, or the Three Cappadocian Fathers as they are named after the place they lived in. Nowadays that is the territory of Turkey. 

Usually the Three Hierarchs were painted full-length, less often, chest-length. In the icon in question the Venerables are depicted side by side, full-length, wearing episcopal omophoria: long scarves embroidered with crosses. Sometimes all the three Saints would be wearing white omophoria. In other icon versions, like in this one, the vestments were, respectively, white, red and blue. The white omophorion was associated with the lost sheep that the Good Shepherd brought home on his shoulders: a symbol of saving the human race by Christ. The red color referred to the blood of Christ that had made satisfaction for human sins. Blue omophorion and vestments were the color of heavenly chastity, innocence and sainthood. Yellow nimbi meant that the Hierarchs had access to Divine Revelation.

St. John Chrysostom, St. Basil the Great and St. Gregory the Theologian are believed to be the Founding Fathers of the Church. All of them were distinguished by bright mind and wisdom, they were learning for their whole life. Therefore, believers pray in front of their icons for success in learning, asking for salvation from impotence and despair, and to set them on the paths of righteousness.