Like I said, Paul uses elder and overseer interchangeably, without gradation.
What do you see as the third permanent office?
At the "embryonic" stage these job-descriptions were not well-defined, and this can be seen in action. For instance, the diakonoi were instituted to wait tables and supervise the distribution of food, so that the Apostles could focus on prayer and the Ministry of the Word (preaching: they didn't yet have New Testament Scripture) but the deacon-evangelist Philip was led by the Holy Spirit to preach to the Ethiopian eunuch ("Ministry of the Word") and deacon-martyr Stephen was martyred for his "Ministry of the Word."
Similarly, the "elders" and "overseers" (presbyteroi and episcopoi) were to some extent distinct, and to some extent synonymous.
It's interesting to see this developing in the next two generations, when St. Ignatius of Antioch (d. 107 AD) wrote his letters to the churches as he traveled to his execution in Rome. In each of these seven letters we can learn something about the nature and structure of the Church at the turn of the 1st-2nd century.
In his Epistle to the Ephesians, St. Ignatius refers to Onesimus (yes, that Onesimus) as the bishop of the Ephesians (c. 1). Then he writes that they should "by a unanimous obedience" you may be perfectly joined and, "being subject to the bishop and the presbytery, you may in all respects be sanctified." (c. 2)
So you've already got a territorial bishop, a "settled" bishop as distinguished from an itinerant, "missionary" bishop.
He speaks of bishops being already established all over the world, writing: "For even Jesus Christ, our inseparable life, is the manifested will of the Father; as also bishops, settled everywhere to the utmost bounds of the earth, are so by the will of Jesus Christ." (c. 3)
Then he writes
“Wherefore it is fitting that you should run together in accordance with the will of your bishop, which thing also you do. For your justly renowned presbytery, worthy of God, is fitted as exactly to the bishop as the strings are to the harp."
So there you have the presbyteroi seen as operating with the "will" of the bishop, "fitted to him" as strings to a harp. They are in a supportive role, subordinated to the bishop and harmoniously coordinated with each other.
There's a whole lot more, of course, Ignatius helps you see that this is all a development from the basic structures set up by the Apostles in Acts/Epistles, and also that the church is defined as the people of God gathered around their bishop.