Can't download NET Framework 3.5 - social.msdn.microsoft.com
...#2. Select .NET Framework 3.5 (include .NET 2.0 and 3.0) check box, press OK and wait for installing, then restart the computer. If the issue persists, I suggest you could use the System File Checker tool to repair missing or corrupted system files. BTW, please make sure you are in a good internet connection. Any feedback will be expected....
https://social.msdn.microsoft.com/Forums/en-US/e919ea32-3581-46e0-995e-40ea5ac896da/cant-download-net-framework-35?forum=netfxsetup