Income Requirement Question

[Lindsey Ambrose]

Hello all, 

I am wondering if the 3x the rent income rule applies to the entire household or each individual tenant. I have 3 friends/roommates looking to move into one of my units and 2 of them make 4x their portion of rent but one only makes 2x her portion of rent as she is a student finishing up school. 

I am worried about that single roommate not being able to truly afford the place. 

Any thoughts?