Installing .NET Framework 3.5 on Server 2019 fails Error 0x800f0950
Mount ISO Windows Server 2019 sources from the sxs folder

Open Command Prompt
dism /online /Add-Package /PackagePath:D:\sources\sxs\microsoft-windows-netfx3-ondemand-package~31bf3856ad364e35~amd64~~.cab.

wait
success.
check.