You need your 2012/2012R2 ISO mounted or unpacked to a folder as the installation media for .net 3.5 is actually included on the DVD/ISO itself!
Next up, you can install by launching a command prompt (As Administrator) and running the following command:
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:d:\sources\sxs
(Replacing d:\sources\sxs with C:\users\you\desktop... or wherever you unpack it to!)
The command switches used above do the following~:
/Online targets the operating system you're running (instead of an offline Windows image).
/Enable-Feature /FeatureName:NetFx3 specifies that you want to enable the .NET Framework 3.5.
/All enables all parent features of the .NET Framework 3.5.
/LimitAccess prevents DISM from contacting Windows Update.
/Source specifies the location of the files needed to restore the feature (in this example, the D:\sources\sxs directory).
This works 99.9% of the time, and if it fails - it usually indicates a problem with the BITS/Windows update service - which are actually both used by the DISM (Deployment Image Servicing and Management) tool. To fix that, take a look at This Microsoft Fixit Article
. The windows 8 manual instructiosn also work for 2012/2012R2!