This assumes all three are on the lease:Those who remain will have to pick up the slack and pay for the lease. Sometimes leaving room mates is unavoidable, but the one who leaves has an ethical financial responsibility to the 2 who remain and a legal one to the landlord.A court might determine after the fact that the legal responsibility applies to the 2 deserted room mates, too. One exception to this could be if the 2 who remain caused the 3rd to leave.Regardless, the landlord is not going to accept 2/3 of the rent and will evict the 2 remaining if they canu2019t pay the rent. He will then probably sue all 3 in court if the rent is not paid in full. So it is to the 3rd partyu2019s advantage to do the right thing here.All three should try as hard as they can to find an eligible and agreeable room mate to take the 3rdu2019s place as soon as possible. And the 3rd should share in the rent expense until the new room mate is found.