Never mind. It turned out that I needed a 'Windows Server 2012 R2' ISO because that's what the target server was; I was trying to a regular 'Windows Server 2012' ISO and that was throwing the 'source not found' error. I downloaded an 'eval' copy of the R2 ISO from Microsoft site, 'mounted' it on the target server and, voila, the following command ran fine and now .NET 3.5 is enabled!
Thanks!
dism /online /enable-feature /featurename:NetFx3 /all /source:d:\sources\sxs