The pronoun them
in its first use in this sentence refers to the sons of Hassenaah
in Nehemiah 3:3. In its second use later in this verse, the reference is to Meremoth.
The implication is that Meremoth did not build by himself but came with a crew under his leadership. This understanding would be valid throughout the chapter.
4 And next to them Meremoth the son of Uriah, son of Hakkoz repaired. And next to them Meshullam the son of Berechiah, son of Meshezabel repaired. And next to them Zadok the son of Baana repaired.