Current Tenant want to add a roommate

Hi - a new landlord here and need some advice. I just rented out a single-family home to two last year in college students ( they are a boyfriend and girlfriend). They did not have a good credit score, but they have a solid co-signer, one of the tenant's parents. It has been a month since they moved, and they asked if they could add a roommate. Should I treat a new roommate as a new tenant and ask for 3x of the monthly rental payment or evaluate a tenant based on their part of the monthly payment (I do not know how they would split the rent among themselves). My requirement is 3x monthly rent and must be paid in one payment each month. I planned to send a pre-qualification letter asking if the person can pay 3x monthly rent payment and additional questions. Please advise how to qualify for a new roommate.