(Diehard vim keybinding user, but editor/IDE nomad) For an expert, I'm not sure that there is anything that st3 has that cannot be achieved by the other two with a combination of plugins and customization.

The biggest difference I have seen is reasonable defaults for a new user. I have lots of friends and coworkers who use Sublime because dropping into vim/emacs is like entering an alternate dimension. They expect to click around and edit text similar like any other program on the computer (which vim lacks) and have a sensible menu system (which emacs lacks). Sublime uses standard system keybindings, so that e.g on a mac Cmd-S saves the current file like you would expect. This is possible to do on both vim/emacs, but not the default. It is really difficult to convince a brand new user that the first step they need to take is opening some random hidden file and customizing their entire editor in a custom configuration language just to get the system default behavior.