offline install.NET 3.5 with Windows 10

If you have the install media, you can use powershell to install offline using the Install-WindowsFeature commandlet. Mount the .iso or insert the disc and then open powershell as an admin and run:

Install-WindowsFeature Net-Framework-Core -source d:\sources\sxs

where d: is the drive letter for the install media.

This microsoft article documents the method, though it is using a network share as the source instead of the install media.

Edit: An alternative method if Install-WindowsFeature is unavailable is Enable-WindowsOptionalFeature. Use the following syntax:

Enable-WindowsOptionalFeature -FeatureName "NetFx3" -Source d:\sources\sxs

Fire up an Administrative command prompt (not PowerShell).

Mount the original installation media.

DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:d:\sources\sxs

Where d is the mounted installation media you used to install Windows.

https://docs.microsoft.com/en-us/windows-hardware/manufacture/desktop/deploy-net-framework-35-by-using-deployment-image-servicing-and-management--dism