Adam Sneller writes:

> I want to call org-update-radio-target-regexp as soon as org-mode
> recognises a new <<<radio target>>> has been created (which seems to
> happen as soon as the third ">" is typed).
>
> What hook can I use to get this done?

I'm not spotting any hook that could be used for this.  The <<<target>>>
fontification happens through a simple 'regular expression => face'
mapping in org-font-lock-keywords.

You're probably aware of this and it's just not as automatic as you'd
like, but just in case: pressing `C-c C-c' with point on <<<target>>
will call org-update-radio-target-regexp.