Reginald Heber (21 April 1783 – 3 April 1826) was a Church of England parish priest who became Bishop of Calcutta in 1823. Heber wrote "Holy, Holy, Holy" for Trinity Sunday, a day that reaffirmed the doctrine of the Trinity and was observed eight Sundays after Easter. The Greek phrase Trisagion translates as "Thrice Holy" – as in this hymn God is described as holy in three different qualities; Agios o Theos means "Holy God". The hymn was first published in 1826.
Heber wrote 57 hymns which were collected by his widow and published in his poetical works in 1842. The tune “Nicæa” was written by Dr. John B. Dykes, a renowned Doctor of Music.
The original hymn (written by Heber) contained four verses, with the first and the fourth ending with “God in three persons, blessed Trinity!” Source: https://www.hymnologyarchive.com/holy-holy-holy
However, another source shows only three verses adapted from Heber's original, with “God in three persons, blessed Trinity!” given only in the first verse: https://www.hymnal.net/en/hymn/h/6
In support of the answer given by brasshat, I found an official Christian Science link which shows how the original words have been changed:
From The Christian Science Hymnal, 1932 edition:
Holy, Holy, Holy, Lord God Almighty,
Early in the morning our song shall rise to Thee.
Holy, Holy, Holy, merciful and mighty,
Which wert, and art, and evermore shalt be.
Holy, Holy, Holy, darkness cannot hide Thee,
Though the eyes of sinful men Thy glory cannot see.
Thou alone art holy, there is none beside Thee,
Perfect in power, in love and purity.
Holy, Holy, Holy, Lord God Almighty,
All Thy works shall praise Thy name in earth,
and sky and sea;
Holy, Holy, Holy, merciful and mighty,
Which wert, and art, and evermore shalt be.
Source: https://hymns.plainfieldcs.com/hymn-117/
As can be seen, the Christian Science version, whilst retaining the words “Holy, Holy, Holy,” removes all reference to “God in three persons, blessed Trinity!” which appeared in Heber’s original, four verse hymn.