Stick red-hot needles under your fingernails, rather than go with Three!
Five years ago, Three blatantly lied to me about sending a replacement phone and sim, when they had not . I cancelled my contract, and after two more months of payment without any service, I was rid of them.
Like a fool, I later took out a "rolling monthly" data contract with Three, after buying a 3G iPad. When I wanted to cancel it after nearly two years, I researched others experiences on the internet, and prepared myself for an ordeal.
On phoning the call centre and trying very hard to appear like a satisfied customer who was likely to buy a new 4G iPad and return to Three after a few months break (no chance!), I had no end of hassle from their call centre staff, for what should have been a simple procedure. (Let me remind you - cancellation of a "rolling monthly" contract.)
It appears that they want an additional one month advance notification to cancel a "rolling monthly" contract, so despite giving notice on 7 January, my last payment will be on 2 March! (Obviously!)
My advice to potential Three customers is: Read all the reviews you can find on them. If all you will ever need is a very cheap, fairly reliable connection, never an upgrade, nor a new sim, nor a new contract, nor to cancel your contract, nor anything resembling an acceptable level of customer service - then, fine, go with Three. If you want anything other than that, don't touch Three, even with a triple-condom-covered, titanium barge-pole! You have been warned!