To install Microsoft .NET 3.5 Features including .NET 2 and 3.0
Pass through the first few steps to add the role and feature until you see .NET Framework 3.5 Features (Figure 1)
Then at the bottom of the window press on Specify an alternate source path (Figure 2)
With your Windows Server 2012 DVD mounted or in the drive locate the sxs folder and copy the location address. In my case its D:\source\sxs (Figure 3)
Now with the alternate file location set go through the install as normal and this time it won’t fail.