Suspect #3 was driving DM and MS's ride home. After DM/MS ditched the truck they would have to return to the Yukon to get home. He was basically the getaway car driver.
Suspect #3 would have had to wait for the other two through the murder of TB, the trip to the farm, the disposal of the body (which could have been several hours for equipment setup and incineration), the caching of the truck...perhaps they even took on the removal of the seats from the truck on Monday night/Tuesday morning
Travel and these activities together might have taken up a good chunk of time...6 hours? Is it conceivable that Suspect #3 stood around with his hands in his pockets all of this time? Or was he needed manpower to move a body, set up the incinerator, work the bobcat, etc.? I think DM/MS would want to hurry up and take care of things and demand Suspect #3 pitched in