If it was the genuine Three chat then it should be fine. Scammers can't do much with just sort code and account details anyway, they would need other info.
If you were upgrading your phone then Three would need to confirm these details to set up a new Device Payment direct debit. Check your Three account to see if the correct upgrade is now showing and check your bank account for a new direct debit appearing.
If you still have any concerns check your bank account daily for any payment going out or direct debit that you don't recognise. Download a banking app to your phone if you don't have one, makes it easier to check often.
