Origin of the Reformed "Marks of the True Church" In Reformed Theology, "the marks of a true church" are often delineated, to the effect of:


Church discipline
Teaching/preaching
Administration of the sacraments


(These are the marks often denoted in systematic theology, but let it be noted that they are not intended to exclude other important marks such as suffering and love.)

A slightly altered formula is seen in Westminster Confession of Faith, Chapter 25 Of the Church, paragraph 4:


  This catholic Church hath been sometimes more, sometimes less visible. And particular Churches, which are members thereof, are more or less pure, according as the doctrine of the gospel is taught and embraced, ordinances administered, and public worship performed more or less purely in them.


I found something similar in Calvin's letter to Cardinal Sadoleto (available in Vol. III of Banner of Truth's Tracts and Letters Calvin volumes, and in a slim booklet edited by John Olin entitled A Reformation Debate, page 63):


  Since there are three things on which the safety of the Church is fonded, viz., doctrine, discipline, and the sacraments, and to these a fourth is added, viz., ceremonies, by which to exercise the people in offices of piety...


Calvin's letter of 1539 predates the Confession by a little over a hundred years, and is the earliest document that I have personally found this doctrine in. However, I find it unlikely that Calvin wrote any new doctrines in his letter to the Cardinal, so I'm wondering where the origin of this doctrine was.

Does Calvin have earlier writings on it? Did earlier Reformers such as Luther, Zwingli or Bucer discuss it? Were there pre-Reformation theologians who discussed it?
Calvin dedicates an entire Book of his Institutes (Book IV) to the Church, and Chapters 1-2 are about the "true church" in which he mentions the marks of a true church.

From 4.1.9:


  Wherever we see the word of God sincerely preached and heard, wherever
  we see the sacraments administered according to the institution of
  Christ, there we cannot have any doubt that the church of God has some
  existence, since his promise cannot fail,
  
  
    "Where two or three are gathered together in my name, there am I in the midst of them" (Matthew 18:20)
  


So here he lays out the first two points you mention - preaching and sacraments, but while he talks about discipline at length in 4.12 &amp; 4.13, he doesn't include it in his "marks" in 4.1 and 4.2. 

Since Calvin's Institutes were published in 1536, before the letter he mentioned, I suspect he had only recently begun to develop the marks in his mind. By the time he wrote the letter in 1539, he'd added the third mark.

However, I dug a little deeper and while browsing The Apostolic Fathers , a collection of letters and sermons by early church fathers, I found a document entitled The Didache, or The Teaching of the Twelve Apostles.  This is likely a composite document, pieced together by early church fathers, and the author is unknown, but it dates somewhere from A.D 50 to 300, so it's well before Calvin (and Augustine, assuming he had anything to say about it)

In Didache are three "marks" (though not declared as distinguishing points; they are given as instructions) of a church (emphasis mine):


  On the Lord's own day gather together and break bread and give
  thanks, having first confessed your sins so that your sacrifice may
  be pure. But let no one who has a quarrel with a companion join you
  until they have been reconciled, so that your sacrifice may not be
  defiled. For this is the sacrifice concerning which the Lord said, "In
  every place and time offer me a pure sacrifice, for I am a great king,
  says the Lord, and my name is marvelous among the nations" (Mal. 1:11,
  14)
  
  Therefore appoint for yourselves bishops and deacons worthy of the
  Lord, men who are humble and not avaricious and true and approved, for
  they too carry out for you the ministry of the prophets and
  teachers. You must not, therefore, despise them, for they are your
  honored men, along with the prophets and teachers.
  
  Furthermore, correct one another, not in anger but in peace, as
  you find in the Gospel; and if anyone wrongs his neighbor, let no one
  speak to him, nor let him hear a word from you, until he repents


The first paragraph, given the stipulation to repent sins before the sacrifice, seems to be talking about the Lord's Supper: sacraments.

The second paragraph speaks of establishing teachers: preaching the word.

The third paragraph gives instructions for how to treat a non-repentant sinner: discipline

Though it doesn't label them marks in so many words, I think this is the closest (and earliest) you'll find to the idea of the three distinguishing factors. I'm fairly certain, however, that Calvin was the one who called them "marks" first.

Ultimately, I think the general idea stems from Acts 2:42


  And they devoted themselves to the apostles' teaching and fellowship,
  to the breaking of bread and the prayers.