My wife is on Three and we have had serious coverage issues in Pheasey and about a mile radius.
After some thinking we have now cancelled the contract and quite a few others have as they have a very fair policy on that. So if you are on Three and having these issues (as I speak the phone shows no coverage) then ring up Three and they will cancel.
Oh and this has nothing to do with the mast by the NISA as the service was fine until about three months ago.
Three have acknowledged that there is a major technical fault in the area.