Tethering on Three's (or most other UK telco providers) unlimited plans is against their terms and conditions. You'll have to switch to a plan that allows tethering or use an add-on. See their Tethering info page for more information. Also see their Terms and Conditions
However some phones will still allow you to tether, I'm surprised it doesn't work on the GS2. Did you buy it from Three? So the tether function still works, but when you do tether a Laptop, they send you a message?
Generally they detect mobile usage that comes from PCs and Laptops and then they ban you. But tethering to other mobile devices seem to work, and also if you keep your data usage low, they may not bother.
Another possible work-around is to try and spoof the user agent and trick Three into thinking that the data is being sent by a mobile device. However, this is not fool-proof and they may still be able to detect, for example downloading a windows update.
Just to be clear, I would still recommend you buy an addon or join a different plan which allows tethering. If you think the Terms and Conditions are unfair, file a complaint or switch telcos.
Information on Three's The One Plan
SIM plans with All-you-can-eat data do not allow tethering (except for the One Plan). If you would like to tether, you will need to choose a Pay Monthly plan and / or an Add-On that does allow tethering.
So it is possible to get the One Plan, for unlimited data and tethering.
And from Tethering Explained
If your smartphone plan includes all you can eat data, then this is for data consumption actually on your smartphone. It does not include using your smartphone as a modem to connect other devices such as laptops and tablets – also known as “Tethering”. Tethering is included in (i) The One Plan; (ii) the One Plan SIM only; or (iii) By purchasing an Ultimate Internet Plan with the Tethering Add on. The add-on can be purchased via My3 on your handset and is also available to customers on our Talk and Text plans.
See their Pay Monthly plans