When you try to install .NET Framework 3.5 from ServerManager, you could run into this problem, that the following message will be displayed, when you install the feature:
“Do you want to specify an alternate source path? One or more installation selections are missing source files…”
For a quick fix, mount the Server DVD installation media, either by ISO or physical media, and type the following in a command prompt:
dism /online /enable-feature /featurename:NetFX3 /all /Source:d:\sources\sxs /LimitAccess
In the above example D: is the DVD-DRIVE, change it to match your setup.
After a short time, you should see this:
And you are good to go 🙂