Trinitarians say there are three persons in one being, or three persons in one God. Although not all trinitarians agree on what word to apply the threeness to and what word to apply the oneness to.
The point from Galatians is that Paul apparently had no conception of “threeness” within God. He made the simple observation that a mediator requires more than one party, whereas God is only one. He had no idea of God being “three” in any sense, and assumed his readers had the same view.