But this is exactly the problem. Is there a “fourth source,” which is human reason?
Yes. We can learn the truth of things through reason…
Indeed, great theologians and philosophers over the ages (Augustine, Aquinas, and John Paul II, to name only a few) have advanced our understanding of the truth, and even you and I participate in this ongoing process. That’s actually in the Catechism section I cited:
The supernatural sense of faith
91 All the faithful share in understanding and handing on revealed truth. They have received the anointing of the Holy Spirit, who instructs them and guides them into all truth.
92 “The whole body of the faithful . . . cannot err in matters of belief. This characteristic is shown in the supernatural appreciation of faith (sensus fidei) on the part of the whole people, when, ‘from the bishops to the last of the faithful,’ they manifest a universal consent in matters of faith and morals.”
93 “By this appreciation of the faith, aroused and sustained by the Spirit of truth, the People of God, guided by the sacred teaching authority (Magisterium), . . . receives . . . the faith, once for all delivered to the saints. . . . The People unfailingly adheres to this faith, penetrates it more deeply with right judgment, and applies it more fully in daily life.”
Growth in understanding the faith
94 Thanks to the assistance of the Holy Spirit, the understanding of both the realities and the words of the heritage of faith is able to grow in the life of the Church:
— “through the contemplation and study of believers who ponder these things in their hearts”; it is in particular “theological research [which] deepens knowledge of revealed truth.”
— “from the intimate sense of spiritual realities which [believers] experience,” the sacred Scriptures “grow with the one who reads them.”
—“from the preaching of those who have received, along with their right of succession in the episcopate, the sure charism of truth.”
95 “It is clear therefore that, in the supremely wise arrangement of God, sacred Tradition, Sacred Scripture, and the Magisterium of the Church are so connected and associated that one of them cannot stand without the others. Working together, each in its own way, under the action of the one Holy Spirit, they all contribute effectively to the salvation of souls.”