Can't say if the third one will stand on its own without the backstory, what with being in the UK and not released yet, but I suspect there'll be references to the older stuff, being such a story-oriented game.
I think the first one is worth playing anyway - it's a good game. Second one I didn't really engage with, and didn't feel so much of an advancement into the story, but I don't remember it being hugely long anyways.
