Help writing a lease for shared tenants

[Zac Boelkow]

I am buying a duplex that has three tenants on one side. There is a husband and wife that take one bedroom and another person (seller of the property) who rents the other bedroom out. They split the rent up like this: Husband/wife pays $400 and the single man pays $250 for a total of $650 for their side. The single man was the owner of the property and just uses the bedroom for a place to crash while in town. The husband/wife have lived here for over 8 years and this is their primary residence. They are on social security and can not afford $650 on their own. The gentleman that is sharing the property with them is just basically helping them out. Through my conversations with him I believe he will stay in the property for another 1-3 years. 

My question is how would I write the lease so in the event when either party leaves that the people who stay are on the hook for the entire months rent of $650... I can see this being an issue for the husband/wife. I can already here them saying "our rent is only $400/month"...