Because the word Holy is repeated three times in it.

Why is this prayer called an angelic song?

Because it was received from heaven from the angels themselves, who sing it before the throne of God.

When and how was this song received from heaven?

In the 5th century, during the lifetime of the pious emperor Theodosius II and Patriarch Proclus, a terrible earthquake occurred in Greece, causing entire villages to fall into the ground. People lived in the streets and squares out of fear. A prayer service was performed in the field for the cessation of the earthquake. During the prayer service, a boy from among the people was lifted up to heaven by an invisible hand. Lowered again to earth, he told the astonished people that he had heard angels in heaven glorifying God with the words: Holy God, Holy Mighty, Holy Immortal.

The people, repeating these words, added to them: have mercy on us. After that, the earthquake stopped.

A Brief Doxology to the Most Holy Trinity

In Russian:

Praise be to the Father, and to the Son, and to the Holy Spirit, now and always, and unto the endless ages. Amen.

Always - always. Century - century. Forever and ever, without end.

To whom is this prayer?

This prayer of praise to all three persons of the Holy Scriptures. Trinity.

Prayer to the Most Holy Trinity

In Russian:

Most Holy Trinity, have mercy on us; Lord, cleanse our sins; Lord, forgive our iniquities; O Holy One, come and heal our weakness for Thy glory.

The Trinity is the three persons of the Godhead. Lord, the Lord is a king, lord over the whole world. Lawlessness is a violation of the law, a sin. Heal - make whole what is damaged, heal. Infirmities - weaknesses, sins.

To whom is this prayer?

To the three persons of St. Trinity.

What do we ask in this prayer to St. Trinity?