After some unsuccessful attempts to turn on .NET Framework 3.5 on a newly installed Windows 10 (even if I have a stable Internet connection) and a failed try to run .NET Framework installer, I found a very graceful way to do this.
1. Open Command prompt
2. Run this command
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:d:\sources\sxs
If everything goes smoothly, you’ll see these lines showing the happy end.
Deployment Image Servicing and Management tool
Image Version: 10.0.10240.16384
The operation completed successfully.