how to get a stm32f3 discovery board into boot loader mode?

@Andrey:
Did you see this note (from here: http://www.espruino.com/Other+Boards):

"Note: This board is more difficult to connect to. You need to power
up the board without 'USB USER' plugged in, and then plug in USB
later. If you subsequently reset the board, you'll need to unplug USB
and plug it back in."

You need to use the "other" USB connector (USB_USER) and not the one you used to upload the Espruino software.
Maybe that is the issue you are running into?
But reading your note again, you probably already tried that.