If you wait to install your Prerequisites after you run Windows Update on Server 2012/2012 R2 you will get an error
I really doesn't matter what you specify as you "Source" the Prerequisites Powershell command we all love will fail.
So if you do searches you will come across others having this issue. They solved it by running.
dism /online /enable-feature /featurename:NetFX3 /all /Source:d:\sources\sxs /LimitAccess
Which would help... but there is one last key piece of information I'll give you... which is the point of this blog because I found the answer in a comment on an obscure blog post.
If KB2966827 or its brother KB2966828 are installed the dism command will fail to install about 67% of the way in.
Not surprisingly the work around is to uninstall either of those two KBs... or to do your Prerequisites before you run Windows Update.
There you go... my pain is your gain. Enjoy.