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If it's a surprise, like Alice and Bob are fighting over the Macguffin, and suddenly Charlie, who has not even been mentioned once in the story up to this point, swoops in to steal it — this could be Did Not See That Coming or possibly even Diabolus ex Machina.
If it's a more general "three sides are openly fighting for the same goal", that's Mêlée à Trois. That trope is not specifically about who wins, but it's probably common that the two sides with the worst vendetta get distracted by each other.
Dark Horse Victory is another possibility, but it is specific to formal contests. Without concrete examples, it's hard to tell if you meant a life-or-death struggle or a competition with judges.
Edited by KD
Third I think is the most fitting, thanks. Though what I mean is mostly "Two sides are pitted against each other, they both are cold about third one at best, and third just uses their rivalry as a tool to win. May resolve into Enemy Mine, if the third side is actually evil.
Dude i could be gaming
Is there a trope for such phrase, when a third party is oppurtunist and wins the game, and the rest looses?
"The Smerfolymippic Games" used this, but I don't think me giving examples is necessary.