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What plan are you on with Three? As far as I'm aware, it's only 'The One Plan' which allows tethering, other plans (even those sold with 'all you can eat' data) don't allow tethering. It might be that Three are enforcing this now.
 
I have an ancient 3yr old on going contract that does allow tether. In fact I used to get text messages saying I have exceeded my allowance (even though its unlimted). Customer Services just told me to ignore them. Never been charged for tethering in all that time. Ring CS and ask them to confirm whats going on. They sometimes are less than honest so just be pushy and threaten to leave if your on the rolling contract. Once you get the 'official' authorisation you'll be fine.
 
been ok for me today too

was using tethering all day with my iPad as the hotspot and my laptop connecting to it. 3 did sent out a tweet earlier today saying some users were having an issue with the service and just to reboot.

[edit] I'm on the 1gb/month rolling contract
 
Im on a plan with three that dosent allow tethering. However i am typing whilst tethered. Sooner or later i will get busted but until then, i will keep utilising it.

It seems you may just of got caught earlier, that is of course if you're on a plan that dosent allow tethering.
 
Update: fixed this. I recently updated to their all-you-can-eat data add-on which blocks tethering. I've removed it now and they've said tethering should become possible again in the next few days.
 
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