After bringing you the last way to get the £15 One Plan earlier this year I'm back (no longer working for Three!) with some more info.
Having visited the Exeter store on the 28th of Nov I was told that the £15 All In One with unlimited internet on payg was stopping soon. Also surprise surprise they had no payg cards in stock (I know that trick after working in a 3 store)
It doesn't surprise me as 3 have been upping their contract pricing lately and are most likely removing unlimited tethering from out of contract One Plans.
I don't have a date but considering the unlimited tethering is rumoured to be removed on the 5th of Jan I imagine it's somewhere around then. Apparently one user from another forum (poor source I know) said the AIO changed to 3gb 'accidently' on their website recently and then changed back, which wouldnt surprise me as what they change it too.
"PAYG RATES THAT NEVER CHANGE" Three said, conveniently not stating the addons wouldn't change