We don't loose today if there are three players, if we totally **** up again we end up with 7, meaning they'd have to kill one of us again. If Sinz is mafia, and he dies, then we still go to 7 players, but that means the mafia would have to kill us for 2 days without us finding another one, which is unlikely considering the agent-sinz link.
If there are 4 players, then yeah, we'd die if Sinz isn't mafia. But if Sinz is mafia, then we can get Agent, resulting in the 2 player thing above. Once we get one mafia, we've cracked them, and can start searching for connections.
We are by no way "screwed". We've had setbacks, but we can easily still win.