At first I thought that there would be no forensic trace of Suspect #3 in DM's car because DM drove it for 4 days after the murder and would have obliterated the prints of whoever was driving it that night.
Then I thought, DM is a taller than average guy, odds are Suspect #3 had to adjust the seat and rear view mirror and these are areas that don't get touched much and prints could be preserved there
MS's buddies are a
pot smoking crowd and pot smokers often also smoke cigarettes. Suspect #3 could be a smoker like MS and could have left butts on the farm, waiting at the side of the road, etc
Some of MS's friends seem to know something, and at least one has been willing to throw AS under the bus (and imply he's more dangerous than MS). That friend says she has called LE to report her suspicions/knowledge
If Suspect #3 kept his cell phone on for that night's journey there would be vague record of his movements
I thought it was interesting that LE finally confirmed the sex of the third suspect on the 22nd: We believe the danger has subsided, Kinsella said, despite a third suspect still being sought, about whom police say they encourage that individual to turn
himself in.
http://www.thestar.com/news/crime/2013/05/22/tim_bosma_hamilton_police_arrest_second_murder_suspect.html
To date LE has made a point of using gender-neutral pronouns, etc. So that is a hint that they have a specific male in mind