What's wrong with my MMR?

Hi everyone. Today I played 3 ranked games, I gained 17 LP from the win and lost 18 from both losses. I was wondering why is my MMR so bad? I don't think that I'm that bad at League, concidering I have a 57% overall winrate. On champions other than Pyke and Thresh my winrate is bad, I know that, but that's the reason I don't play them. Can you help me figure out why my MMR is so bad?
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