Using your phone as a modem and connecting it to other devices through WiFi or USB to get onto the internet (also called 'tethering'), is not what the plan you're on was designed for. If you would like to speak to someone about tethering options available to you, please call 333 free from your Three phone. In the meantime if you want to get your phone back to browsing the internet you'll need to turn it off, allow it to reset itself for a few moments, and then turn it on again. If you're looking for a Mobile Broadband connection, you'll get a better, cheaper and faster experience on one of our Mobile Broadband tariffs or devices. Just go to your nearest 3Store.
So it seems that if you have got yourself a new plan or upgraded your current plan, you will not be allowed to have tethering unless you pay Three an extra sum. Checking with their customer service, the price for this feature comes at £5 per month which is quite a lot considering you already have "unlimited" data usage on your phone.
Fortunately, you do not need to pay them extra for service as you can easily bypass their system that detects if you might be tethering or not.
How to bypass 3 network tethering block
1. Download and Install "User Agent Switcher" extension for your browser on your computer.
For Google Chrome, there are many to choose from and for Mozilla Firefox, you can download from here.
2. Once you have installed the extension, you need to select any mobile device such as iOS (iphone) or Android from the user switch agent options.
3. Connect your computer to the wireless hotspot.
4. Enjoy accessing internet from your computer.
Additional notes on using "User Agent Switcher"
This trick will work on almost all devices. No extra configuration is required on your phones other than enabled the wireless hotspot.
Sometimes you might notice that the tethering block warning would pop up again and it is likely because an application in the background (for example Steam) is using the internet and to prevent it from happening, you would need to close that application.
You do not need to restart your phone if you get the tethering block, simply restart the data connection on your smartphone.
Please check your service agreement before you try to bypass three's tethering block :)