I don't believe 3's statement. ISPs are required by law to reveal who was using a particular IP at a certain time upon Court Order.
Understand they may be required, but if X buys a second hand dongle on Ebay from Y (from Z from a shop in their area, sold to the shop by A, who was given it by B at work) how would Three know who 'X' is.
SIMs with 3 GB of data pre-loaded are sold on Ebay and it means that even though Three may have the IP address and 'mobile number' they are still no closer to identifying 'X'. Not sure how easy it would be to determine the IMEI (?) number and which property the device was being used in, either.
If you run a business, have a second ISP and backup web hosting...