I prefer #3.From a blind-usage pure logical standpoint, the user would expect the left-most (un-highlighted) button to abort the ongoing process entirely, without consequence.
It is the same as having the Escape key at the far left-top of the keyboard - intuitiveness.
The grouped buttons convey a special relationship, meaning they are modifications on a theme. "Save" and "Don't Save" have this relationship, in that both buttons will (in all likelihood) result in the terminating of the application - with either saving the results, or discarding them.
The outlier, then, is the cancellation of this process - which goes against the flow - so it would be expected to be placed separately.
--The loon