Given the earlier reference to “God and…the Lamb” (both on “the throne,” Revelation 3:21), we might expect here a plural pronoun: “them.” The singular pronoun indicates the unity of the two Persons referenced by the terms “God” and “the Lamb.” Here is an implicit allusion to the Trinity.
3 No longer will there be anything accursed, but the throne of God and of the Lamb will be in it, and his servants will worship him.