Ask your roommate to reverse positions, i.e., to stand in your shoes while you stand in theirs. This can sometimes be the most effective way of getting your point across and, contrariwise, understanding where your roommate is coming from. Avoid blaming each other. Assessing blame often has the effect of making the other party defensive and anxious to find fault with you, widening instead of narrowing the conflict. 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 psycho-analyze your roommate. Most people don't like the feeling of being analyzed.