While a three gauge set would be nice from a collector viewpoint I see little practical use for them (other than a Skeet set) which is undoubtedly why there are so few of them. When you put smaller gauge barrels on a larger gun you either wind up with a heavier gun or one with an unpleasing appearance due to the steep taper down to the forward parts with an unnecessarily wide spacing of the barrels.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra