Thank you, Space. Your reply spurred me on to dig further.
If I’m already signed in to AudioJungle, and I go to the VideoHive of PhotoDune sites, I’m taken to the homepage. At top right, there’s the ‘Sign In’ button, as if I’m not already signed in, plus the usual ‘Start Selling’ and other buttons.
If I click the start selling button, it starts the whole sing-up process again.
But what your queries led to try was to click the Sign In button - and, voila, it automatically signed me in to the other sites. Very good. In fact, now that I’ve done that once, I can click on any of the three sites and remain signed in. Excellent.
So now I have two new issues!
Uploading - Once signed in, my username is displayed at top right. Clicking on that shows a drop-down menu. On VideoHive and Audioungle, that menu includes an Upload option. No such option in the menu when signed in to PhotoDune.
Profile - All three, on the Dashboard tell me “fill out your profile so our buyers know who created the items they’re buying…”, and I’d like to do that. But when I click on the 'Take me to my profile" button, the profile editing page that comes up has only a field for avatar upload. There are no other fields to ‘fill out’. Should there be?
I’ve tried on a couple of different browsers just in case it’s a compatibility issue - but it’s not.