Tenant breaking their lease

[Michael Poloncic]

Three individuals signed a lease together, all friends, the lease is set up where each of them are expected to pay 1/3 of the rent. Two of them decided to move out in December without informing me. One of them accidentally paid for January rent because of auto pay and is now requesting it back. I informed them that their contract states they are expected to pay up until the end of the agreement regardless of living their or not. 

Now, this tenant is saying that they don't remember any agreement and thought they paid month to month and could leave whenever. I am now under the suspicion that their friend, the one who is still living at the property, is the one who created separate accounts for all three of them and did the signing for each individual who is under this lease agreement. 

I'm trying to prepare incase this is true. What is the recourse for the tenant who signed the lease under each individuals name? And, would that invalidate the contract? Or are the other two tenants still bound to the contract regardless?

Any additional input would be great. Thank you!