In my case I need to automate the installation of .NET 3.5 within a MDT 2013 build and capture task sequence for Windows 8.1

In the State restore add a step to set the WindowsSource Path
The Windows Source will be located within the Operating Systems folder on your Deployment Share

1. Add a Set Task Sequence Variable step
   1. Type Task Sequence Variable: WindowsSource
   2. Type Value for your source files: %deployroot%\Operating Systems\Windows 8.1 x64 Enterprise VL\Sources\SXS

1. Add a Install Roles and Features step
   1. Select your Operating System within the step
   2. Mark .NEt Framework 3.5 (Includes .NET 2.0 and 3.0)

Is it that simple? Yes!

NOTE: If you want to use a Run Command Line to install it, you could use something like this: DISM.exe /online /enable-feature /featurename:NetFX3 /All /Source:”%deployroot%\Operating Systems\Windows 8.1 x64 Enterprise VL\Sources\SXS” /LimitAccess