.NET Framework 3.5 cab format offline installation package download:
http://pan.baidu.com/s/1c2F3Fzy
Put the downloaded offline installation package named NetFx3.cab in the C:\Windows folder of the system disk.
Then run the command prompt as administrator, type and hit enter to run the following command:
dism /online /Enable-Feature /FeatureName:NetFx3 /Source:"%windir%" /LimitAccess
Installation is quick, about 2 minutes.
Installation screenshot:
