Three charges for third party payment without my knowledge

Hi, I’m in England East Midlands , I’ve got a problem with three mobile. For months they’ve been charging me 3x my phone bill. When I finally realised and contacted them about it I’ve been told it’s a third party payment which i apparently agreed to but I definitely did not. It was text messages from something called office scanner which I was sure is a scam. Three are saying they can’t do anything to refund me the money and the other company is not picking up. What can I do here?