I first opened the three images in Nikon Capture NX2 and set the WB into Direct Sunlight. I then adjusted the "Picture Control" to "Landscape" to give them added contrast and sharpness. I adjusted the exposure compensation to decrease the middle image by 0.87 stop and the left image by 1.67 stop. The three images are now all in exactly the same condition. I then saved them as non-compressed TIFE files. Because I knew that I would do several further modifications of the images in Photoshop and Capture NX2, repeated compression in JEPG format would certainly degrade the quality of the images.
I opened the three images in Photoshop and photomerged them into a panoramic image. The merged picture showed clearly a prospective distortion, since she shot the image from an upward position. The distortion was corrected using Filter>Distort>Lens Correction of Photoshop. I re-cropped the image, the center upper part of the sky had a gap, I refilled it with the Stamp tool. I now saved the panoramic image in TIFE and reopened it in Capture NX2.
I used Shadow Protection to give the forest at the lower left side of the image more light and played around the Curve to further improve the overall contrast and saturation of the image. Now it is almost done, I saved the image, reopened it in Photoshop and applied the final Smart Sharpen (141%, 0.2 pixel, lens blur, more accurate) step as recommended by Ken Rockwell. When looking at 100%, there is some noise in the shadow area, since we did not use any noise reduction either in camera or during the post-processing. It is typical for the the Nikon D90/D300. The full-frame D700 would be better. But I doubt that I will see this noise in the final print.
The final image in non-compressed TIFE is 176.3 MB or just over 30 MP. I can make a non-interpolated print of 300 dpi at 24.7 x 13.9 inches. If not using the Photomerge and the computer softwares, we would need a medium format digital camera with a PC lens to get an image with such detail.