You need to use :bdelete 3.c
or in short form :bd 3.c
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
:buffers
or :ls
or :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 :help windows.txt