It's simple. In this text (a baptismal context IMO), they are witnesses
of our confession of the Father and the Son and the Holy Spirit (in accordance with Matthew 28:19-20), thus they witness the dedication of our lives to God, thus our our baptismal confession (which is
also normally done "in the presence of many witnesses," i.e. the congregation cf. 1 Tim. 6:12), and also (from the continuing context of EP 13.1) they are
also witnesses
of our conduct as we continue to keep or observe the commandments (i.e. Matthew 28:20 "teaching them to observe all the things I commanded you").
Clement of Alexandria
“Selections from Prophetic Writings” (Eclogae Propheticae) 13.1
Πᾶν ῥῆμα ἵσταται ἐπὶ δύο καὶ τριῶν μαρτύρων, ἐπὶ Πατρὸς καὶ Υἱοῦ καὶ Ἁγίου Πνεύματος,
ἐφ' ὧν μαρτύρων καὶ βοηθῶν αἱ ἐντολαὶ λεγόμεναι φυλάσσεσθαι ὀφείλουσιν.
“Every verbal statement [Or: “matter” “point spoken”] is to be established upon the basis of [Or: “at the evidence of”] two and three witnesses,” [Deut. 19:15 (Clause-B, paraphrased) LXX] in the presence "of a Father and of a Son and of Holy Spirit," [Matthew 28:19 (Clause-B, articles omitted)] before whom, as witnesses and as helpers it is our duty to keep what are called 'the commandments'.”
Compare the IMO dedicational and baptismal context of 1 Timothy 6:12
1 Timothy 6:12
ἀγωνίζου τὸν καλὸν ἀγῶνα τῆς πίστεως, ἐπιλαβοῦ τῆς αἰωνίου ζωῆς, εἰς ἣν ἐκλήθης
καὶ ὡμολόγησας τὴν καλὴν ὁμολογίαν ἐνώπιον πολλῶν μαρτύρων.
"Fight the good fight of the faith. Take hold of the eternal life to which you were called
and about which you made the good confession in the presence of many witnesses."
Further proof that it is a paraphrased quotation (omitting the definite articles) from Matthew 28:19, compare also the article-less Greek quotation of Matthew 28:19 in the Excerpta Ex Theodoto:
Clement of Alexandria
“Epitomes from the writings of Theodotus and the so-called Eastern teaching from the time of Valentinus”
Chapter 76:3-4
Καὶ τοῖς Ἀποστόλοις ἐντέλλεται· Περιιόντες κηρύσσετε, καὶ τοὺς πιστεύοντας
βαπτίζετε εἰς ὄνομα Πατρὸς καὶ Υἱοῦ καὶ [4.] Ἁγίου Πνεύματος, εἰς οὓς ἀναγεννώμεθα
"And he commands the disciples 'When ye go about, preach and them that believe baptize in the name
of the Father, and of the Son and of the Holy Spirit" in whom we are born again..."
("Excerpta Ex Theodoto" prepared by Robert Pierce Casey, "The Excerpta ex Theodoto of Clement of Alexandria"
Studies and Documents 1, London, Christophers, 1934, Pages 40-91)
“And to the apostles he gives the command. Going around preaching ye and baptize those who believe
in the name of Father and Son and Holy Spirit...” (Conybeare, Page 987)
NOTE: These are the only two translations of the text I could find. Casey adds the definite articles in the English which are absent in the Greek. Conybeare rightly omits the articles in English in his translation (I could do my own but I can't be bothered today).
Compare also the article-less Greek quotation of Matthew 28:19 in the Didache:
The Didache
Chapter 7:1-4
Greek Text, from “The Teaching of the Apostles - Newly Edited, with Facsimile Text and a Commentary,
For the John Hopkins University, Baltimore, From the Ms. of the Holy Sepulchre,
(Convent of the Greek Church) Jerusalem,”
By J. Rendel Harris, 1887
Facsimile of manuscript Folio 78a Codex Hierosolymitanus, Page 117 (113)
https://babel.hathitrust.org/cgi/pt?id=hvd.hnnc3z;view=1up;seq=129
Περὶ δὲ τοῦ βαπτίσματος, οὕτω βαπτίσατε· ταῦτα πάντα πρειπόντες, βαπτίσατε
εἰς τὸ ὄνομα τοῦ Πατρὸς καὶ τοῦ Υἱοῦ καὶ τοῦ Ἁγίου Πνεύματος ἐν ὕδατι ζῶντι. [2.] ἐὰν δὲ μὴ ἔχῃς ὕδωρ ζῶν, εἰς ἄλλο ὕδωρ βάπτισον· εἰ δ’ οὐ δύνασαι ἐν ψυχρῷ, ἐν θερμῷ. [3.] ἐὰν δὲ ἀμφότερα μὴ ἔχῃς, ἔκχεον εἰς τὴν κεφαλὴν τρὶς ὕδωρ
εἰς ὄνομα Πατρὸς καὶ Υἱοῦ καὶ Ἁγίου Πνεύματος. [4.] πρὸ δὲ τοῦ βαπτίσμος προνηστευσάτω ὁ βαπτίζων καὶ ὁ βαπτιζόμενος καὶ εἴ τινες ἄλλοι δύναται· κελεύεις δὲ νηστεῦσαι τὸν βαπτιζόμενον πρὸ μιᾶς ἢ δύο.
"Now concerning the baptism, baptize in this way: after saying in advance all these things, baptize
“in the name of the Father and the Son and the Holy Spirit” [Matt. 28:19] in running water.
[2.] But if you do not have access to running water, baptize in another kind of water; and if you are not able to [baptize] with cold water, baptize in hot water.
[3.] But if you possess [neither], pour water onto the head three times
“in the name of a Father and of a Son and of Holy Spirit” [Matt. 28:19 paraphrased].
[4.] Now, before the baptism, the one baptizing and the one being baptized, and if any others are able, let them fast! Now, you must urge the one being baptized to fast for one or two days before.”
The Excerpta Ex Theodoto 7.3-4 quote is literally the book before, and from the exact same manuscript as Eclogae Propheticae 13.1, (the only complete manuscript these works which they are found in - if my memory serves me rightly - but don't quote me on that just in case). In the Didache you also have Matthew 28:19, both with the articles and without the definite articles in the Greek.
The Greek is the exact same article-less construction of Matthew 28:19. There is no mistaking!
EP 13.1 Πατρὸς καὶ Υἱοῦ καὶ Ἁγίου Πνεύματος
ET 7.3-4 Πατρὸς καὶ Υἱοῦ καὶ Ἁγίου Πνεύματος
DD 7.3 Πατρὸς καὶ Υἱοῦ καὶ Ἁγίου Πνεύματος
Conclusion, Eclogae Propheticae 13.1 is definitely an article-less quotation of Matthew 28:19, and not 1 John 5:7 (Clause-C, KJV-numbering), and they are "witnesses"
of us, or whoever is meant by those who keep the commandments in EP 13.1 (cf. Matthew 28:20 "teaching them to observe all the things I have commanded you"), not (
testimonium dant) witnesses
to us, of their own one-ness.
To reiterate again, this text (EP 13.1) is pseudo-Christian Gnostic quackery (weird stuff), but it's simply not referencing the Comma.