There are 2 Unitarian readings that I can find.
1. Jesus is the temple
The referents (in square brackets) according to this exegesis:
Behold, I [God the Father] am going to send My messenger [John the Baptist], and he will clear the way before Me. And the Lord [the Father], whom you seek, will suddenly come to His temple [Jesus]; and the messenger of the covenant [Jesus], in whom you delight, behold, he [Jesus] is coming,” says Yahweh of Hosts.
But instead of the Trinitarian reading that Jesus is heading off to the stone temple in Jerusalem, a more proper reading is that
the Lord coming to his temple refers to the Spirit of God and His glory descending into the Temple.
Jesus did not refer to the Temple in Jerusalem as "my Temple." He referred to it as "my Father's house" (John 2:16). Furthermore, the Temple to which God came was not the stone temple but the body of Jesus Christ. God the Father tabernacled in the human flesh of Jesus when the Spirit of the Father descended upon him. For that reason, Jesus referred to the Father abiding in him. Jesus was the Temple of God. In the same way, the Bible informs us that we, Christians, the body of Christ, are the Temple of God because the Spirit of God dwells in us.
2. Jesus visited the stone temple as agent of YHWH
"The Lord" visited the stone temple in the person of YHWH's appointed agent, Jesus the messiah. This alternate exegesis is from Biblical Unitarian Podcast Episode 138: Malachi 3:1 and the Preexistence of Jesus. See the notes here. Quote from the notes:
Based on what we know about how messengers functioned in the ANE and the principle of agency, we can already begin to interpret this passage. First of all, a messenger represents the sender in the fullest possible way. The agent carries the authority and often the prerogatives of the sender, and since the sender is Yahweh himself, we can expect this messenger to bear considerable authority. Second, since the messenger carries forth a message from Yahweh himself, we should not be surprised if the messenger acts like, sounds like, looks like, or is even described like Yahweh. However, it is very important to note that while the agent represents his sender to the highest degree, the two persons remain distinct and they should never be collapsed into one person. The messenger is not Yahweh himself, but rather fully represents Yahweh. Third, the Hebrew noun malak and the equivalent Greek noun angelos render into the English noun “messenger”, and a messenger could be a human messenger or it could refer to a heavenly messenger, often angelic in nature. Herein lies some of the ambiguity of the passage, but we are in no rush to come to a conclusion on that matter. What we can say for certain is that Yahweh of hosts, according to Malachi, will send his messenger and that this messenger will clear the way before Yahweh.
If I had to take a guess, I would say that the messenger of the covenant is likely the same messenger referenced at the beginning of the passage. Both are described with the same Hebrew noun for messenger, and it is not abundantly clear that the author wants the reader to regard this messenger as an additional messenger, thus making two.
In regard to the Lord who will come to his temple, my initial reaction is to regard this as a reference to Yahweh, since the temple is Yahweh’s. I am also inclined to think that the Lord is a reference to Yahweh himself because the verse indicates that the Lord whom you seek will come to his temple, indicating that, although the second temple had been built for around a 100 years or so, depending on the dating of Malachi, God had not entered into the temple as we see with God entering Solomon’s temple. Malachi seems to be indicating that the Lord, meaning Yahweh, will come to his temple, but his coming will be predicated on the arrival of a messenger, namely the messenger of the covenant.
Now, some interpreters think there are more than two persons referenced in Malachi 3:1. Since the text is ambiguous, there could be a reference to more than two persons. But this would require an explanation of how the temple belongs to a Lord who is distinct from Yahweh himself.
I also want to point out that the messenger mentioned in Mal 3:1, whom I regard as one and the same as the messenger of the covenant spoken of in that same verse, is a figure that Yahweh is going to send. But this expression of sending does not demonstrate that this messenger is already alive already. It simply indicates Yahweh’s intention, as the participle with the verb shalak indicates in Hebrew. In other words, the text of Mal 3:1 does not state that this messenger is already alive, and suggestions that this messenger is the preexistent Jesus are inferring something that the Hebrew simply does not state.
The episode further says that no specialist of Malachi (the notes cited 5 commentaries) identifies the promised "messenger of the covenant" as the preexistent Jesus, but probably Elijah (clue: Mal 4:5-6). NT does "regard John the Baptist as the prophet who comes in the spirit of Elijah", who DID prepare the way for
["the Lord"] to visit the temple, and the Lord of hosts visited the temple through an agent, the authorized and anointed Son of God. Jesus, functioning as a fully empowered agent of Yahweh, fulfilled the promise that Yahweh would visit the temple. For Jesus to function as a sent agent of Yahweh, the gospel writers are exhibiting a very high human Christology.
But according to Biblical Unitarians, this very high human Christology is not high enough to be the preexistent Word as taught by non-Unitarians.