I just re-watched the debate from 2003 between then-Jehovah's Witness apologist, Greg Stafford (author of Jehovah’s Witnesses Defended) and Reformed Baptist James White (available online here). During the audience Q&A, one of the audience members argued that the locution הָאָדוֹן ("the Lord") in Mal 3:1 is a divine title, and ergo, Jesus is numerically identical to the "One God" in the Trinitarian understanding thereof, as the NT figures who fulfill this text are John the Baptist (the messenger) and Jesus (the Lord).
Firstly, while used of Yahweh elsewhere in the Hebrew Bible, it is a rare locution, only appearing five other times in the Old Testament, all in Isaiah:
Therefore saith the Lord (הָאָדוֹן), the Lord of hosts(יְהוָה צְבָאוֹת), the mighty One of Israel, Ah, I will ease me of mine adversaries, and avenge me of mine enemies. (Isa 1:24)
For, behold, the Lord (הָאָדוֹן), the Lord of hosts(יְהוָה צְבָאוֹת), doth take away from Jerusalem and from Judah the stay and the staff, the whole stay of bread, and the whole stay of water. (Isa 3:1)
Therefore shall the Lord (הָאָדוֹן), the Lord of hosts(יְהוָה צְבָאוֹת), send among his fat ones leanness; and under his glory he shall kindle a burning like the burning of a fire. (Isa 10:16)
Behold, the Lord (הָאָדוֹן), the Lord of hosts(יְהוָה צְבָאוֹת), shall lop the bough with terror: and the high ones of stature shall be hewn down, and the haughty shall be humbled. (Isa 10:33)
And the Egyptians will I give over to the hand of a cruel lord; and a fierce king will rule over them, says the Lord (הָאָדוֹן), the Lord of hosts(יְהוָה צְבָאוֹת). (Isa 19:4)
In each of these instances, הָאָדוֹן is coupled with "Lord (Yahweh) of hosts" (יְהוָה צְבָאוֹת), something one does not find in Mal 3:1, so there is a difference between the Malachi text and the Isaiah texts that have this locution. The coupling of הָאָדוֹן with יְהוָה צְבָאוֹת is the reason why we know Yahweh is in view; not the bare of use הָאָדוֹן.
Furthermore, what undermines this text from Malachi as being a valid "proof-text" is that there is a distinction between Yahweh and the figure labelled הָאָדוֹן as evidenced by the fact that it is Yahweh who is speaking (cf. 2:17) about a person distinct from Him who is הָאָדוֹן:
Behold, I will send my massager, and he shall prepare the way before me: and the Lord (הָאָדוֹן), whom ye seek, shall suddenly come to his temple, even the messenger of the covenant, whom ye delight in: behold, he shall come, saith the Lord of hosts (יְהוָה צְבָאוֹת)
While not disproving Trinitarian formulations of Christology and concepts of ontological oneness, etc., this argument does not prove such.