Just as I thought, the fine print answers your question (funny how that works!).
It has NOTHING to do with round trips or not - they just used that as an example. The fine print says:
"Purchase and fly at least six (6) Delta or Delta Connection® marketed and operated flight segments between April 1 and June 30, 2011, and earn Silver Medallion status through February 29, 2012. Purchase and fly at least 12 Delta or Delta Connection marketed and operated qualifying flight segments between April 1 and June 30, 2011, and earn Gold Medallion status through February 29, 2012. Codeshare flights operated by carriers other than the Delta Connection carriers are excluded from this offer."
So you need 6 SEGMENTS, not three round trips. A segment is a distinct flight number. So, as said above, if you are booked on the same flight number in both directions, it is one segment (argue all you want, but you will probably lose).
Where do you see the offer that says you need three round trips by the end of the year? The offer mentioned above says you need 6 segments by JUNE 30, 2011 (which is the equivalent on 3 round trips, but you don't need all round trips to qualify). Maybe you are talking about a different offer. It is always good to post links to the offers you are referring to.
You quoted the wrong offer.
http://dmn.delta.com/ace_offers/earlyplatinum/