This is not easy. You could be on the hook for rent for the rest of the lease with your option of recovering it from the other two roomates. You may want to send them written notice, certified return receipt, warning them of the problem and that it must immediately cease as they have breached the lease and placed you in an unsafe environment. They must cease their conduct immediatley upon receipt of the letter. If they don't clean up their act within a few days, you may want to talk to the landlord about the condition. Be careful since, as I said before, you could be on the hook for the entire amount of the lease. Your roomates could be evicted. However, at some point you may want to give notice (and it should be in writing and certified return receipt mail) that you will be moving out of the apartment if the condition is not remedied within a certain number of days (I would say 3-5 days should suffice) due to the unsafe condition within your apartment.
If the landlord is reasonable and lets you out... get it in writing. This is a very, very difficult situation. The moral is to always get a written agreement with your roomates and be very careful with cosigning anything. If you do, you may want extra protection if the cosignees are not trustworthy.
