Method 1 - Try The Installation Via Server Manager
Typically, the easiest way to install Net Framework 2.0, 3.0 & 3.5 is to go to Server Manager...
1. Open "Server Manager"
2. Click on "Manage"
3. Select "Add Roles and Features"
4. On the "Before you begin" page click "Next
5. On the next page, ensure that "Role-based or feature-based installation" is selected, and again click "Next"
6. Select your local server out of the list and continue to click "Next"
7. Bypass the "Server Roles" page by clicking "Next" again
8. Onto the "Features" page now, put a "check" in the "Net Framework 3.5 Features" box and again click "Next"
9. Select "Install" to begin the installation
Method 2 - Specify Alternate Source Files If Method 1 Fails
If the installation fails, you can specify the alternate source path/files which are contained on your Windows Server 2012 / 2012 R2 installation disc or .iso
1. Insert your installation disc or mount your .iso within Server 2012 / 2012 R2
2. Open "Server Manager"
3. Click on "Manage"
4. Select "Add Roles and Features"
5. On the "Before you begin" page click "Next
6. On the next page, ensure that "Role-based or feature-based installation" is selected, and again click "Next"
7. Select your local server out of the list and continue to click "Next"
8. Bypass the "Server Roles" page by clicking "Next" again
9. Onto the "Features" page now, put a "check" in the "Net Framework 3.5 Features" box and again click "Next"
10. This time, click on "Specify an alternate source path" and type the path of your Server 2012 or 2012 R2 installation media e.g.
D:\Sources\SxS (Where D:\ is your DVD Drive containing the installation disc)
11. Select "Install" to begin the installation