To cancel a mobile phone contract you have to send them a letter dicating that you want to cancel.
Most mobile phone companys will not allow you to do it over the phone but a few of them will as long as you give a minimum of 30 days notice. I have never heard of having to give as much notice as they have requested from you.
I would write a letter to Three and also copy the same letter to Oftel, may put the wind up them to settle early.