.NET Framework 3.5 needs the source files off the Windows Server 2012 DVD in order to install. You can do this online and without needing a restart using DISM.
1. Go to a command prompt and enter this:
dism /online /enable-feature /featurename:NetFX3 /all /Source:d:sourcessxs /LimitAccess
Note: Source should be the Windows DVD location, ie d: /Source:x if x is your DVD mount point.