1. Create a new folder on the C drive. In this example, I'll call it Update.

2. Copy the file microsoft-windows-netfx3-ondemand-package.cab from the Windows 10 disc or iso to this folder. It's in the sources/sxs folder. 

3. Go to a command prompt (probably needs to be an administrator command prompt)

4. Type this command, replacing “C:\Update” with whatever you called your folder: 


dism /online /enable-feature /featurename:netFX3 /all /Source:C:\Update /limitaccess


Explanation: 

dism is a tool for working with Windows images. It stands for "Deployment Image Servicing and Management".
 
“/online” tells it to modify the currently running instance of Windows
 
“/enable-feature” tells it we want to enable a feature
 
“/featurename:netFX3” tells it that we want to enable the .Net 3.5 framework
 
“/all” enables all parent features of our feature 
 
“/Source” tells it were to find the feature 
 
“/limitaccess” tells it not to try to find the update on Windows Update or WSUS