Avoid blaming each other. Whose fault it was is irrelevant when everyone agrees to work towards a solution. Be task oriented in sticking to the topic. Avoid digressing into other non-related grievances or anecdotes. Talk about actions which can be changed, rather than personalities.
Personal attacks destroy communication of productive ideas.
Don't team up with one roommate against another. This creates defensiveness in the third roommate. You are all working together for a solution. Don't psychoanalyze your roommate. Most people don't like the feeling of being analyzed.