This is very usefull, when the title of a page is to long for the size of the browser tab of the panel page. Then a special panel icon (the website icon changed in color) can inform the user about the displayed context.
I once investigated this myself, and if I’m not completely misreading
my understanding is that K3 does not have such option (the favicon URL appears to be hardcoded into the Panel template).
What you could do (apart from possibly adding this enhancement idea on the new feedback.getkirby.com wishlist tracker) is to add a custom Panel JS script and change the favicon by manipulating the DOM? (This would actually enable some cool tricks – for example turning the favicon orange when there are unsaved changes, etc.)
If Kirby 3 has less functionality than Kirby 2, then I suggest you stick with Kirby 2 and be happy. Or use another CMS altogether. I’m pretty sure Bastian will refund the licence price if you are so unhappy with it that you have to complain in bold letters because of a minor feature.