I have analyzed my last 25 games using tracker to see which ones I was playing with a 3 stack on my team. Raw data is included at the end but here are the numbers

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Win percentage: 54%

This is across all of my games which is to be expected.

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Win percentage when I have a 3 stack on my team: 36%

An 18% drop from my normal win rate but that doesn't tell the whole story because some of those 3 stack wins are against another 3 stack on the enemy team

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Win percentage when I have a 3 stack on my team and opposing team has a max 2 stack: 22%

This is pretty damning. These games are unwinnable because the 3 stack refuses to communicate and almost always 2 of the 3 are so blatantly boosted that it is obvious they don't belong in that elo

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Win percentage when I don't play with a 3 stack: 81%

You can see the raw data here for yourself. it looks like the biggest predictor for the outcome of my game is if I am unfortunate to be matched with a 3 stack.

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Being matched with a 3 stack while the opposing team don't have a 3 stack makes the game completely unbalanced and I hope riot stops matching 3 stacks against non 3 stacks