248-264AD: Dionysius of Alexandria refers to 1 John 5:7 and quotes part of it:

👉Greek: Εἰ τῷ τρεῖς εἶναι τὰς ὑποστάσεις, μεμερισμένας εἶναι λέγουσι, τρεῖς εἰσι, κᾂν μὴ θέλωσιν, ἢ τὴν  θείαν Τριάδα παντελῶς ἀνελέτωσαν. (Dionysius of Alexandria, Epistolae II Dionysii Episcopi ad SS Dionysium. Fragmenta. Migne Latin, PL 5.128A-B) Link

👉English [Against the Sabellians]: if they maintain that a separation is necessarily involved in the expression ‘three Hypostases,’ yet there are three - whether they admit it or no - or they must completely destroy the divine triad. (Dionysius of Alexandria, Epistle to Dionysius of Rome, Fragment. Translation found in Basil ... . The Holy Spirit. 29 (72). < Link >.)

👉The words "there are three" (τρεῖς εἰσι) are from 1 John 5:7's words "there are three".
Also, the words "three Hypostases,’ " (τρεῖς .... ὑποστάσεις,) and "divine triad." (θείαν Τριάδα) refer to 1 John 5:7's "For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost,...".