Should be held accountable for there shocking service

had a mobile contract with them 2 years ago when it finished I went with someone else it took me 4 weeks to get rid of them and get my pac number from them after numerous telephone live chats I ended everything with them last week I checked my bank statements to find that they have still been taking taking my dd My fault should of check the statements I know anyway I called the bank to cancel and they gave me a refund of £349 tonight I get a email from three saying that I owe them £349 and they are sending debt collectors to me as I haven’t paid my bill !!! Slightly strange as this is the first email I’ve had from them in two years !!! And a week after I’ve cancel my direct debit ???if anything they owe me money !! I’m shocked that it’s took 2 years to contact me if they think I owe them money and one email threatening debt collectors I’m in the middle of the battle with them and being passed from pillar to post and waiting in que after que trying to sorting this out two years after I cancelled my contract if you google it you find 100 of 3 customers all has similar experience with this shambles of a company there customer service is shocking they lie constantly I will advise how I get on I find all this so unsettling and upsetting and I’m not letting them get away with the way I’ve been treated I shall keep you posted !! But please please If your thinking of going with 3 mobile don’t like me you will end up regretting it years later ! Someone seriously needs to be held accountable for this company’s actions