hreflang attributes are in place, aren't they?

It took some time to get them properly in Google Webmaster Tools 0:)

At a glance, I can only see them in place on your respective homepages, where it only has limited value.

It should ideally be across all pages of all sites, so as to declare all 'sibling' pages, e.g.

https://www.fortsu.co.uk/running-shoes/all/trail

https://www.fortsu.de/laufschuhe/alle/trail

https://www.fortsu.com/running-shoes/all/trail

you can also use XML sitemaps to implement it, which tends to be preferable, especially if you don't want the extra bloat in the HTML of your pages.

I will double check category pages (those you point out) but I am sure hreflang attributes are in place for product pages ;)

I will check the XML sitemap stuff, thanks for the tip.