If we start with I Timothy 5:17, a verse that speaks of "elders who rule" and "elders who labor in the Word and doctrine" ("elder" = presbyteros), it would not be surprising that we would conclude that the elder is an ordinary officer in the NT church, and that there are two kinds of elders: the ruling kind and the teaching kind. Then we simply add deacons, stir, let cool, and voila! we have a three-office ecclesiology (or at least 2 1/2).
But it is not altogether obvious to me that this is the most prudent approach. I Timothy 5:17 is a single verse whose context doesn't really have anything to do with the specifics of church office. In fact, it is not certain whether Paul is talking about church offices at all (when you look at the entire chapter, particularly its discussion of fathers, young men, brothers, old women, mothers, young women, sisters, and widows, it is at least possible he is using presbyteroi to refer literally to old men, not the office of elder).
If we seek to understand the nature and number of church offices, therefore, we should begin by looking to I Timothy 3:1-7 and 8-13. When we begin there, we see that Paul lists bishops and deacons with no mention of elders at all (cf. Phil. 1:1). When the clear passages illuminate the less-than-clear ones, it becomes possible, if not likely, that I Timothy 5:17's "elders who labor in the Word" are bishops (or ministers), and its "elders who rule" are deacons (or servant rulers).
So do me a favor and try this experiment: Read Paul's description and qualifications for the deacon, doing your best to forget everything you know about how that officer functions in the modern Reformed church. What we see is that the deacon must be dignified, he must hold to the deep things of the faith, and he must rule his house well.
Kinda sounds like what we expect from a "ruling elder," doesn't it?