Regarding tethering on 3, they now have systems in place to detect this and people with the £3 all you can eat add-on are reporting being sent to a holding page when trying to thether.
They only seem to be policing this on the new £3 add-on for now from what ive read online, but this could change at any time.
3 allow tethering on the One plan and the Internet Max 2Gb add-on for other plans only...
Cheers