Adam Clarke on Galatians 4:6

And because ye are sons - By faith in Christ Jesus, being redeemed both from the bondage and curse of the law; God - the Father, called generally the first person of the glorious Trinity, hath sent forth the Spirit - the Holy Ghost, the second person of that Trinity, of his Son - Jesus Christ, the third person of the Trinity - crying, Abba, Father! from the fullest and most satisfactory evidence that God, the Father, Son, and Spirit, had become their portion. For the explanation of the phrase, and why the Greek and Syriac terms are joined together here, see the notes on Mar 14:36, and on Rom 8:15 (note).