Watch out Three have changed this in the last couple of months. The old "one plan" gave you unlimited tethered data, the new one is 2GB of tethered data. I suspect too many people were going wire free and hammering the network.
Most months I barely beat 1-2GB but when I'm working on site it can be 20GB which means I may end up stuck on an overpriced "with phone" contract and may have to buy unlocked devices again. I'll have to see what the exact details are in a few months time.
I can't find an official word from Three about this, but I think this doesn't apply to existing plan holders.
I've done about 50GB/mo tethering last month and this with no ill effects yet. I'll be holding on tight to this plan for as long as possible, although as I'm on the rolling, 1-month contract version I suppose there's nothing stopping them from shifting me to a different plan apart from good will.
Most months I barely beat 1-2GB but when I'm working on site it can be 20GB which means I may end up stuck on an overpriced "with phone" contract and may have to buy unlocked devices again. I'll have to see what the exact details are in a few months time.