Two of the buttons are safe to press and one (discard) is very dangerous. What you are used to on Windows aside, I don't think it's great to put the dangerous button in the middle. Such a danger sandwich increases the likelihood that you miss a safe button and hit discard. Seeing a trash can first should wake you up, no? :p
One thing that is interesting is the danger button has a white trash can, while the cancel button uses red. In this dialog, cancel is actually a safe action, so we should probably emphasis the trash can danger more.
FWIW, OS X, Gnome (a Unix desktop environment), and others use guidelines where the cancel button is put leftmost. The reasoning being that, reading left to right, is that button will get more and first attention. Spine doesn't do that, though I think a cancel, save, discard button order could make sense (but of course still would not match Windows).
I personally think the examples you have shown are terrible. Not because of button order, but because Yes / No requires the user to read and understand the question very carefully. Save / Do Not Save is also not great because putting the word "save" on a button that discards all your work is not a good idea.
Interestingly, Windows guidelines are OK or Yes, No, Cancel, Apply, Help. I wonder why they like Apply right of Cancel.
I know you are used to Windows, so the various guidelines or any amount of good reasoning isn't going to change which button you go for instinctively. Changing the button order would probably cause catastrophes for Spine users who are used to it though. The best we could do is leave it as is and make it configurable. Gnome has a setting for an alternate button order, so it is not unprecedented.
