For everyone who is still struggling with this incredibly unhelpful process from Three which almost seems designed to dissuade you from cancelling them, this is how I did it.
Firstly, I did this on the actual website rather than the app because I had thought that maybe the site would provide more functionality. There was no option on the home tab as stated above to"manage add-ons" so unless Three is purposefully trying to misdirect people, I don't know why you have advised that. I had to go onto the "PLAN" tab, rather than the home tab. Then after scrolling down past your plan, contract, device, there is a section for purchasing more add ons. Counter-intuitively, if you go in to buy more add ons, it will show you the add ons you already have. Next to them, there should be an option to remove them. I should also say, that I did this on the website rather than the app. On the app, it looks like the 'Plan details' section in the home tab is where you can "Buy Add-ons", so if you click that link it should take you through the same process.
Please for the love of god Three just change the title of that link to "manage" rather than "buy" add-ons and you will save a lot of customers the time and hassle when they are trying to cancel. Something in me tells me that's baked into the process though....
