Basic Trope: Battle between no less than three fractions with each trying to defeat other two.
Straight: Super Troper Team is in the middle of a fight with Empress Abyss and her army of alien soldiers, when suddenly Emperor Evulz appears with an army of undead and orders them to kill both factions.
Alternatively, while Super Troper Team and Empress Abyss haven't actually agreed to an Enemy Mine, Emperor Evulz happened to teleport in right between them, so they end up focusing most of their effort on his undead anyway while still making pot shots at each other.
Double Subverted:...only to double-cross their new allies in a middle of the battle, so as to defeat both sides.
Parodied: Emperor Evulz and Empress Abyss join forces, then remember "Oh, right, we're evil!" and resume fighting as before.
Zig Zagged: Super Troper Team and Empress Abyss team up when Emperor Evulz appears, until the Empress comes down with Chronic Backstabbing Disorder. Seeing how Empress is now the strongest single faction, the Emperor agrees to an Enemy Mine with Super Troper Team, only to betray them himself when the playing fields are level again... but Super Troper Team saw it coming and fights him off, returning it to a straight Mêlée à Trois...
Averted: Super Troper Team fights Empress Abyss, and when Emperor Evulz shows up, he decides to wait to see who wins before joining the fight.
Enforced: "Another fight between good and bad guys? It's been done to death. We need to add something here, like, I don't know, a third party, who wants to kill everybody, or something.”
Lampshaded: "Sure, fighting with one evil army wasn't enough, now we have two. At least they don't join forces.”
Invoked: Lord Overseer directs Emperor Evulz into the battlefield where Super Troper Team and Empress Abyss are fighting, knowing everyone will be too spiteful and shortsighted to form any alliances.
Exploited: EterniCorp Military and Medical manipulates the three sides (all of which buy from EterniCorp) in order to bring this about as often as possible, ensuring constant demand for their products and a stable balance of power between them.
Defied: Bloodgun finds out that Emperor Evulz is going to interrupt a battle between Super Troper Team and Empress Abyss, so he attacks the Emperor's troops and forces them to retreat before they can interrupt the battle.
Discussed: "Emperor Evulz has been moving his forces through this region. We might be in trouble if he decides to throw his hand in — but at least Empress Abyss will be just as badly off."
Deconstructed: The weakened and wounded heroes are no match for Emperor Evulz's undead, as are Empress Abyss' troops. Yet, out of mutual pride and hate, they refuse to join forces and decide to keep fighting on two fronts, instead on focusing on one enemy. They are quickly overpowered and killed.
Reconstructed: While both heroes and aliens are weakened and refuse to join forces, they are still strong enough to fight both Emperor Evulz and each other.