I've been haveing problems installing the .Net 3.5 also. The error i get from Windows Update is:
0x80070011
I tried opening and Admin cmd and used:
dism.exe /online /enable-feature /featurename:NetFX3 /Source:f:sourcessxs /LimitAccess
where f: is my dvd drive where the install disk is located.
I also tried:
dism /online /cleanup-image /restorehealth
but non of these were able to resolve the issue.