just to clear up a few things, you can not together on a pay as you go or pay monthly contract with all you can eat data, unless you are on the one plan OR have bought the internet Max add on which allows tethering on any contract plan for £5.11.
If you tether on any plan other than those mentioned above, you will be redirected to a 3 page which says you can't use your internet connection for tethering. However, it may be possible to tether for a short amount of time before three realise and redirect you to this page.
On the one play, ayce data is not capped in bandwidth or speed, tethering is allowed. There is technically a soft cap @450gb Max per month, (used to be 80gb when ayce was introduced). If someone happens to use this much data in one month, which is near impossible, the guys at three will investigate your account and see if you're using the data for personal use or illegal use. If its personal use they will reset the soft cap. If they find out its non personal use they have the right to cut you off.
But like I said, no one will ever use 450gb in one month.