Commentary Par XIV 28-29

Lombardi (DDP Lombardi.Par.XIV.28-32) points out that none of the earlier commentators had revealed the plan in the first two lines, which is to set the "one" of the first verse against the 'three' of the second, and the 'three' of the first against the 'one' of the second, thus making dramatic the relations of the Trinity, one-in-three as well as three-in-one.  He also discusses the significance of the parallel relation between the two 'two's in the lines, representing the human and divine united in the Second Person of the Trinity.  Porena (comm. to these verses) summarizes what is presented here succinctly: 'Theological designation of God, who lives and reigns eternally as a single Substance, two Natures, and three Persons.'
It took a bit longer until the palindromatic structure of these verses was understood as reflecting Joachim of Flora's structure of history, with its three great Ages: the first, of the Father; the second, of the Son; the third, of the Spirit.  See C.Par.XIV.67-78.  And see one of the 'additional drawings' in the Liber figurarum (Reev.1972.1), for the three overlapping circles representing the three Ages.  See also Dronke (Dron.1975.2), pp. 7-9.

Longfellow (DDP Longellow.Par.XIV.28)points out that this tercet is imitated by Geoffrey Chaucer in the last stanza of his Troilus and Criseyde.

Moevs (Moev.2005.1), p. 27 and n. 34 (p. 199) points out that there was current amongst medieval theologians (followed by some Dantists today) the notion that the Trinity was in a place apart from (and higher than) the Empyrean.  It was known as the coelum Trinitatis (the heaven of the Trinity) and in it God in His three persons lived apart from manifestations of His creation.  Moevs is doubtless correct in asserting that Dante simply combined these two heavens in his Empyrean.