Share your experience!
Are you using an original Nano SIM or you did cut a bigger SIM yourself or using a trimmer at a shop?
We need to exclude SIM defect here, so can you test another SIM in your phone?
Well if possible you can buy a new SIM from Three, this way if the problem goes away then we know it's the old SIM, if not then we can suggest it's a hardware issue. In this case you'll need to contact your Local support and get your phone examined.
Have u fixed this problem coz I have exact same issue like urs