Before accepting swordsman's advice it's worth bearing in mind that he often has the role of "strongest player" in these scenarios .

What swordsman3003 said. That is exactly what I would do.

Start suiciding to whoever is the strongest player, and put up a maximum defense against whoever is the weakest.

If the strongest player accepts your offer and you freely give that player some centers, or if it even LOOKS like that might really happen, the weaker players may get nervous and the alliance could collapse. Or the stronger player may attempt a solo win (which you should directly state you will facilitate and then start doing so).

You should make it crystal clear to the weaker players that you prefer to die to a solo win than to be edged out of a draw, and they only have a turn or two to give up the attack on you before you throw the game. You should do something that makes is manifest that you are willing and able to throw the game, like ceding multiple centers - especially key centers - to the stronger player who is attacking you.

If this plan doesn't work and the players' patiently pick you apart according to a fair partition (and the strongest player holds back), then probably there's nothing you could have done.

You lose.

Claeser offers solid advice, of course, and if you're really lucky it might even work, but in the scenario you've described you really need to have started some pretty serious diplomacy some time before.

When everything is looking good, it usually means you've missed something important

Talk to all of them. Convinve them to reconsider. Make them wary of each other. If one of your enemies stabs another, that's two less to worry about.

Make someone see you're more useful alive than dead.

if everry power youre not at war with and you didnt stab that exact turn attackes you, and you are not on the way to solo but having a decently sized empire, what do you do?