Several factors:
1. Rent needs to be counted for 10 mon, not 12 month.
2. What is avg rent if renting to regular family in the area? i.e. Non-student. Sometimes you cannot expect all houses will be rented by students.
3. management. If you rent the house by room, you have a lot of management issue, because you are dealing with 3 students instead of 1 family.