How to repair python3.5 and why is it broken at all?
Check to see if you got the link command the right way round. What you wrote in your question is correct, but if you had the source and the target the wrong way round when you typed the link command, you could have clobbered the executable. You could also have cobbered the executable if you ran the linking command while in the  ...3.5/bin directory.

Check that the executable still looks right with 

ls -l /Library/Frameworks/Python.framework/Versions/3.5/bin/python3.5


If you have accidentally clobbered it then re-installing is probably the easiest fix. 

Using the -f flag is potentially dangerous, since it will replace files without warning. It's good practice to run commands like ln rm and so on without the -f flag, at least initially.