What does mismatch factor value mean in Panoweaver?

Mismatch factor is used to evaluate on what extent the images are stitched together. The smaller the mismatch factor value, the better. It is nothing to do with color and brightness blending.
Generally the value for stitching 3 fisheye images will be higher than stitching 2 images since there are more stitching lines in stitching 3 fisheye images.