Here is a good statement from Perkins on the third use of the law in his commentary on Matt. 5:18:
"If it be further said that the law itself is abrogated; for that every one that breaks the law is not accursed according to the sentence thereof (Deut. 37:26). Answer. We must know that the law is but one part of God's word, and the gospel another, revealing another part of God's will, besides that which the law made known; for it adds a qualification to the law, moderating the rigor thereof, after this manner: He is accursed (saith the law) that faileth in any commandment, except (saith the gospel) he be reconciled again in Christ, and in him have the pardon of his transgressions. And yet the law remains forever a rule of obedience to every child of God, though he be not bound to bring the same obedience for his justification before God."