We have an ArcGIS Server Advanced license which gives us certain extensions such as Network Analyst, 3-D Analyst, and Spatial Analyst.
It looks like they are not bound to the same license as the ArcGIS Server Advanced, but can be installed on another machine that only has ArcGIS Server Standard licensing.
I believe I was able to do it in a previous version, but never used the extensions but I am curious about doing so when we upgrade to ArcGIS Enterprise 10.5.1
Although you would be eligible for the said extensions of ArcGIS Server with an Advanced license, the license codes could be different for each extension. Please contact your local Esri representative to avail the license codes for the extensions and authorize them.