Alternatively, check the Registry: HKLM\SOFTWARE\Microsoft\NET Framework Setup\NDP\v3.5 should exist.
You inadvertently booted into Windows Recovery Environment (WinRE) or used the wrong DISM environment. Solution: Boot normally into Windows, then run the command. If you are in WinRE, mount your OS partition first or use the Windows installation media directly. Microsoft-windows-netfx3-ondemand-package.cab Download
Enterprise deployment and automation
Enable-WindowsOptionalFeature -Online -FeatureName NetFx3 -Source "D:\sources\sxs" -LimitAccess Microsoft-windows-netfx3-ondemand-package.cab Download
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /Source:D:\sources\sxs /LimitAccess Microsoft-windows-netfx3-ondemand-package.cab Download
The exact filename varies by Windows version and build. Common patterns: