3 domain names for one instance of Drupal 8
I am doing something very similar to migrate some D5, D6 and D7 sites to one D8 site. 
Currently I have this solution:

(1) Have one master domain that serves all content (i.e. proreos.de).

(2) Have some paths (routes) that serve special content for different domains (i.e. /asde/* -> serve content for anwalt-seiten.de/*)

(3) Have ThemeNegotiatiors (ThemeNegotiatorInterface) watching for that special routes to set the 'other' theme (like asde-theme).

(4) Have inverse proxies (apache2 mod_proxy) on the "other" domains that deliver the subcontent from the master domain (i.e. anwalt-seiten.de/index.html -> proreos.de/asde/index.html)

The advantage I hope to have from this approach: 

(a) Have all data in one place (proreos)

(b) Have presentation separated from model (special themes)

(c) Have scalability (by inverse proxies).

But I don't have real experience cause I am still developing. Hope it helps anyway.