When there’s one of me you’re wrong

When there's one of me you're wrong

When there’s one of me you’re wrong

Can you solve this tricky riddle?

When there's one of me you're wrong riddle

When there’s one of me, you’re wrong

when there’s two of me, there’s twenty

when there’s three of me, it’s dirty.

What am I?

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ANSWER

The letter X.