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As far as I can see it has to do with the rotation of the images. Not all images cover the same area of the sky. For example, if you rotate image 2 to match the orientation of the image 1 you will see that it gets cut off where your left-side gradient-line is.
In other words, some parts of your final image have stacked only 1-night worth of data, some parts have stacked 2-nights worth of data while the rest (middle section) has all 3 nights of data.
In DSS can you try to go to stacking options and on the first page of stacking parameters select "Intersection mode" instead of "Standard mode". I have a feeling that will fix the problem, although your final image will be much smaller.
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