I have the following set up in my network.
1. ArcGIS Server and Portal for ArcGIS installed on a machine (A) with Linux OS
2.Oracle DB installed on another machine (B) with Linux OS as well
3. ArcGIS for Desktop installed on another machine (c) with Windows OS
I have created the geodatabase in ArcGIS Desktop that connect to the Oracle DB on machine (B) and it worked.
But, I am wondering if it is possible to add the ArcGIS Server connection from Desktop even if the DB is not installed on the same machine as ArcGIS Server. Will that work? Is it a possibility to have all the 3 installed on 3 different machines?
Yes. It is possible.
You might want to configure ArcGIS Web Adaptor, if not already done. Getting started with ArcGIS Web Adaptor—ArcGIS Web Adaptor (Linux) Installation Guide (10.6) | ArcGI...
Also register your database with ArcGIS Server.
And add a Server connection