By default, .Net Framework 4.5 got installed in Server 2012 where some of the applications like Lync and IIS Sites requires .Net Framework 3.5. So we should not install it by downloading it from internet as it is already available as a feature and we need to install the Feature by selecting Source Path SXS Folder. it will be available in the Server 2012 installable ISO Image under path Source\SXS.
You can copy only SXS folder to local computer and map it as source or you can use Mount Server 2012 ISO and use the path D:\Source\SXS. Here D is the Drive Path
Select the Source path