The Thrice Holy (HOLY God, HOLY Mighty, HOLY Immortal...) Prayers are chanted before the Prokimen and Epistle Reading. The Thrice Holy Prayers are also part of the usual begining of all our Orthodox prayers.
a short hymn consisting of the words 'Holy God, Holy and Strong, Holy and Immortal', sung three or more times
(Gr., "thrice-holy"): liturgical invocation: "Holy God, holy and strong, holy and immortal, have mercy on us."
Literally, "Thrice Holy." The biblical Trisagion, "Holy, Holy, Holy," is the hymn of the angels before the throne of God (Is. 6:1-3; Rev. 4:8), and is one of the most important hymns of the Divine Liturgy. In the Tradition of the Church, this hymn has been amplified into the Trisagion frequently sung during services and said during prayers: "Holy God, Holy Mighty, Holy Immortal One, have mercy on us." The biblical use of "Holy" three times is an indication of the three Persons in the Godhead.
The Thrice-Holy Hymn sung at the Divine Liturgy and often said as a Prayer: "Holy God, Holy Mighty, Holy Immortal, have mercy on us." TROICHEN - Hymn to the Holy Trinity.
Three prayers said before a funeral in the Eastern Orthodox church.
The Trisagion ('Thrice Holy') is a standard hymn of the Divine Liturgy of the Orthodox Christian and Eastern Catholic Churches which use the Byzantine Rite, chanted immediately before the Prokeimenon and the Epistle Reading. In the Orthodox Church it is also included in a set of prayers named for it, called the Trisagion Prayers, which forms part of numerous services (the Hours, Vespers, Matins, and as part of the opening prayers for most services).