While trying to install .NET 3.5 you might stumble on a known issue (bug according to Microsoft) where installation will stop and ask you for an alternative source.
The easiest way (if you dont have the installation media) is to do the following steps.
1) Start the local group policy editor.
2) Go to Computer Configuration -> Administrative Templates -> System
3) Open “Specify settings for optional component installation and component repair
4) select “Enabled” and check the box “Contact Windows update directly……”
5) Click Apply
Make sure windows update is on “Automatically” or “Manual” and then run the following command in Command Promt
dism.exe /online /enable-feature /featurename:NetFX3 /all
That should install .Net 3.5 after a few minutes.