I wonder if we can configure ArcGIS for server or Portal with Web Adaptor installed in different machine. I went through different blogs reaching to a conclusion that we can configure with web adaptor in the same machine.
Configure server with your Web Adaptor in different machine
Question asked by Mithun.Prabhakargistec-esridist 
on Jan 12, 2016
Latest reply on Jan 12, 2016 by mike.robb
on Jan 12, 2016Latest reply on Jan 12, 2016 by mike.robb
Hello Mithun,
If your Web-Server machine is different from ArcGIS Server machine, then the web-adaptor should be installed in the web-server machine. The web-server and ArcGIS server should be able to communicate with each other.
Configuring ArcGIS Web Adaptor after installation—Installation Guides (10.3 and 10.3.1) | ArcGIS for Server