Install .net 3.5 on Windows 10
We do this with a zipped file and batch file command.
We zip up the sxs folder for .NET 3.5 and the batch file command.
the command xcopies the sxs file to a local sources temp folder C:\ITTEMP\Sources\ (with /E /Y switches) for example.
after the copy is done, it runs a DISM command to install .NET 3.5 from the local source.
DISM.exe /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:C:\ITTemp\Sources\sxs
If you have any difficulty get that done. This works very well for us.