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
. . . 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.