You need to use
:bdelete 3.c or in short form
There are a couple of terms you need to understand:
A buffer is the in-memory text of a file.
A window is a viewport on a buffer.
A tab page is a collection of windows.
So, what you're trying to do is close a buffer, not a window. The available commands to manipulate buffers are
:files : See the list of opened buffers
:badd : Add a file to the buffer list
:bdelete : Remove buffer from the buffer list
:bwipeout : Like
:bdelete, but really delete the buffer.
And many more. You can find all about them from