Your roomies only need the permission of your landlord in order to break the lease. They'll also need some funds to pay the landlord for the breach.
However, even your landlord can't force them to stay, if they desire to leave.
You have no ability to do anything, except try and persuade them not to break the lease.
If they do leave, and you stay, you'll have to pay their share (assuming they refuse).
Yeah, the landlord can sue them.
If they don't pay, he's gonna come after you.
I suggest you be prepared for them to bolt.
You can't force them to stay or pay!
You can only try to beg, plead, cajole, barter, or convince them not to leave!