I downloaded the .net framework 3.5 from Microsoft and tried to install it but it won't install on Windows 10. My system doesn't have access to the Internet because the wireless adapter is not working, that's why I downloaded the framework and tried to install it manually.
This is probably because your machine is wanting to go out to the net to get the install media for the .net framework.
If you have your windows 10 media available, try the following in a command prompt (run as administrator)
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:d:\sources\sxs
(where d:\ is your windows 10 media)