A near miss was reported near S27 in spite of three involved aircraft making proper CTAF reports.

Narrative

This morning I departed S27 flew over the nearby mountains and returned across 'lower valley' toward S27; slowing descending to pattern altitude of 3;800 MSL. Approximately four to five miles southeast of S27; I heard an aircraft calling Kalispell City; 'five to the west;' altitude unknown. I then reported my position; 'three to four to the southeast; over the river; inbound for landing; will be entering a left downwind for Runway 13.' At this time a third aircraft reported 'inbound for landing; will keep an eye out for you once I find him.' My course was a forty five degree intercept to left downwind; as reported; and offered a geographic reference as well; 'over the river.' I then reported to both aircraft that I was entering a left downwind for 13; and was nearly to the radio towers southeast of the field. Suddenly my passenger and I noted a red and white aircraft climbing; banking toward us; and taking evasive maneuvers with a turn to the pilot's right. I immediately banked right as well; passing perhaps within 300 FT horizontally of this third aircraft. Based on my position reports; it remains unclear to me why he was in this area; seemingly flying the pattern backward and/or on an intercept to inbound standard left pattern traffic. Now established on the left downwind for 13; I noted the sock was slightly favoring 31. I reported this; and indicated I would be crossing midfield to enter a left downwind for 31. The initial pilot was in sight; and he was in a more favorable position to land first. As such; we both communicated; and I indicated I would land 'number two' and extended by left downwind appropriately. The red and white aircraft that nearly intercepted us reported that he would be 'number three; both inbound aircraft in sight.' After landing; we taxied to the FBO. The third aircraft taxied the length of the runway before returning to 31 to depart. To answer the question as to what I might have done differently; I would have reported my position at five miles; rather than three to four miles. With two clear position reports and a geographic reference; I do not believe that other pilots familiar with S27 would have strayed toward the most common inbound arrival pattern.