Another post of mine describes why we need to enable .NET framework 3.5 manually on windows 8, and all methods to enable it. Now I will tell you what if you are unable to enable it. I am going to take only very specific case, may not be applicable everywhere.
I tried to enable it using DISM command (I had my windows8 ISO image mounted on I: drive)
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:I:\sources\sxs
But it didn’t work, In such case I tried a lot of other options but could not find any solution. Finally I tried removing /All and /LimitAccess in above command and I worked 🙂 So you can also try just the following command
DISM /Online /Enable-Feature /FeatureName:NetFx3 /Source:I:\sources\sxs