I have an existing Jagex account, with two accounts on it, I've recovered my third one, but while trying to upgrade *it* to that Jagex Account, I get told I already have a jagex account at that email...
...how do I add my third account (this one) to my existing Jagex acct so they're all in one place?
Adding another character to your existing Jagex Account is done from inside account management.
Click on 'My Account' at the top of this page then on the blue 'Jagex Account Management' button.
Then the blue 'Characters' button then the blue 'Import Existing' button.
Support article: https://help.jagex.com/hc/en-gb/articles/12270690574993-Add-a-RuneScape-character-to-your-Jagex-Account