If your lease is with three people, all three of you are jointly and severally liable for the monies due on the rent unless the rental agreement is divided literally to only have each of you responsible for 1/3. I believe it most likely the former occurred in your lease, which means each of the remaining two of you is responsible for the total rent and you would need to seek compensation/contribution by suing the third when you find her. As to the issue of the third tenant, you need to see if your landlord will allow you to surrender your current lease, and sign with a third tenant so that the new tenancy is now valid and you take the brunt of finding the former tenant for a lawsuit.