What is the difference between Magisterial teaching and Sacred Tradition in Roman Catholicism?

As Peter pointed out the Church's authority rests upon Sacred Scripture, Sacred Tradition, and Magisterial teaching/interpretation.

Apostolic Tradtion is the root source of Christian revelation. Christ entrusted the message of salvation and doctrinal authority to the Apostles, who in turn handed Christ's revelation through apostolic succession (a.k.a. Sacred Tradition).

The CCC defines Tradition:

THE APOSTOLIC TRADITION

75 "Christ the Lord, in whom the entire Revelation of themost high God is summed up, commanded the apostles to preach the Gospel, which had been promised
beforehand by the prophets, and which he fulfilled in his own person and promulgated with his own lips. In preaching the Gospel, they were to communicate the gifts of God to all men. This Gospel was to be the source of all saving truth and moral discipline."

In the apostolic preaching. . .

76 In keeping with the Lord's command, the Gospel was handed on in two ways:

orally "by the apostles who handed on, by the spoken word of their preaching, by the example they gave, by the institutions they established, what they themselves had received - whether from the lips of Christ, from his way of life and his works, or whether they had learned it at the prompting of the Holy Spirit";

in writing "by those apostles and other men associated with the apostles who, under the inspiration of the same Holy Spirit, committed the message of salvation to writing".

. . . continued in apostolic succession

77 "In order that the full and living Gospel might always be preserved in the Church the apostles left bishops as their successors. They gave them their own position of teaching authority." Indeed, "the apostolic preaching, which is expressed in a special way in the inspired books, was to be preserved in a continuous line of succession until the end of time."

78 This living transmission, accomplished in the Holy Spirit, is called Tradition, since it is distinct from Sacred Scripture, though closely connected to it. Through Tradition, "the Church, in her doctrine, life and worship, perpetuates and transmits to every generation all that she herself is, all that she believes." "The sayings of the holy Fathers are a witness to the life-giving presence of this Tradition, showing how its riches are poured out in the practice and life of the Church, in her belief and her prayer."

79 The Father's self-communication made through his Word in the Holy Spirit, remains present and active in the Church: "God, who spoke in the past, continues to converse with the Spouse of his beloved Son. And the Holy Spirit, through whom the living voice of the Gospel rings out in the Church -and through her in the world - leads believers to the full truth, and makes the Word of Christ dwell in them in all its richness." (CCC, Sect. 1, Chapt. 2, Para. 75-79)

The word Magisterium is just a fancy word for "teaching office." Magisterial teaching is guided by the Holy Spirit to accurately interpret both Tradition and Scripture. The Magisterium is necessary for Sacred Scripture to remain authoritative. Without an authoritative interpreter to interpret Scripture, then Scripture loses its authority. Any one could interpret it anyway they wish. Perhaps this is why there are thousands of denominations. This is analogous to the necessity of the U. S. Judicial branch interpreting the Constitution in order to prevent total anarchy that would result from the private interpretation of U. S. citizens. However, unlike the Judicial branch, the Magisterium is guided by the Holy Spirit.

The CCC also emphasizes the necessity of an authoritative interpreter to defend and protect the authority of Sacred Tradition and Sacred Scripture:

THE INTERPRETATION OF THE HERITAGE OF FAITH

The heritage of faith entrusted to the whole of the Church

84 The apostles entrusted the "Sacred deposit" of the faith (the depositum fidei),contained in Sacred Scripture and Tradition, to the whole of the Church. "By adhering to [this heritage] the entire holy people, united to its pastors, remains always faithful to the teaching of the apostles, to the brotherhood, to the breaking of bread and the prayers. So, in maintaining, practicing and professing the faith that has been handed on, there should be a remarkable harmony between the bishops and the faithful."

The Magisterium of the Church

85 "The task of giving an authentic interpretation of the Word of God, whether in its written form or in the form of Tradition, has been entrusted to the living teaching office of the Church alone. Its authority in this matter is exercised in the name of Jesus Christ." Thismeans that the task of interpretation has been entrusted to the bishops in communion with the successor of Peter, the Bishop of Rome.

86 "Yet this Magisterium is not superior to the Word of God, but is its servant. It teaches only what has been handed on to it. At the divine command and with the help of the Holy Spirit, it listens to this devotedly, guards it with dedication and expounds it faithfully. All that it proposes for belief as being divinely revealed is drawn from this single deposit of faith."

87 Mindful of Christ's words to his apostles: "He who hears you, hears me", the faithful receive with docility the teachings and directives that their pastors give them in different forms.

The dogmas of the faith

88 The Church's Magisterium exercises the authority it holds from Christ to the fullest extent when it defines dogmas, that is, when it proposes, in a form obliging the Christian people to an irrevocable adherence of faith, truths contained in divine Revelation or also when it proposes, in a definitive way, truths having a necessary connection with these.

89 There is an organic connection between our spiritual life and the dogmas. Dogmas are lights along the path of faith; they illuminate it and make it secure. Conversely, if our life is upright, our intellect and heart will be open to welcome the light shed by the dogmas of faith.

90 The mutual connections between dogmas, and their coherence, can be found in the whole of the Revelation of the mystery of Christ. "In Catholic doctrine there exists an order or hierarchy of truths, since they vary in their relation to the foundation of the Christian faith." (CCC, Sect. 1, Chapt. 2, Para. 84-90)

Tradition is the source of both Scripture and the Magisterium. The New Testament wasn't completed until the end of the 1rst cent., and wasn't canonized until the end of the 4rth. The Gospel was transmitted by Tradition and was interpreted by the Magisterium long before the canon was approved by Pope Damasus I. From then on the Church has been supported by those 3 legs.