just wondering: how do you know that you actually need this particular version of .net framework?
is there a certain program that depends on it, to the best of your knowledge?
usually, if a program needs a certain version of .net framework, and you don't have it installed, the program will tell you straight away, and even offer to install it for you.
So if you aren't getting any pop-ups like that, maybe you don't even need it.
EDIT: A few days ago, I tried to install a ISO of win10 AU that had ".NET Framework 3.5 Enabled" on it, and I had nothing but grief from this ISO. The installation would grind to a halt at 99%.
When I tried a different ISO, without .NET, everything went smooth, and has been running smooth since then. That's why I say you maybe don't even need this headache of a .NET, and might be better off without it.