Austin picks up the discussion on the doctrine of the Trinity by looking at each person, the eternal Father, the eternal Son, and the eternal Holy Spirit. Looking at the three drives us back to the one as we remember that God has revealed himself as one God in essence and three in person. These personal subsistences in God can be distinguished, but never separated.
Recommended Readings:
Ecumenical Creeds
Belgic Confession
Westminster Confession of Faith
Delighting in the Trinity, Michael Reeves
Systematic Theology, Louis Berkhof
Reformed Dogmatics, Herman Bavinck