Here is my theory based on generalizations I've come up with myself. You may or may not agree.

I think that current flyfishing community falls into the 2 piece crowd, and the multi-piece crowd.
The multi-piecers tend to want the rod to break down as small as practical for storage/transport. As technology has improved, multi-piece rods can be produced with better and better actions, so it seems 3 and 4 piece blanks are becoming 4, 5, and 7 piece blanks.
The 2-piecers don't want a bunch of ferrules in their rods because of its impact (real or imagined) in the action and 2 piece rods are also a bit less expensive than the multi-piece equivalent.
3 piece rods don't really serve either camp well and I would suspect that they aren't produced because demand is very low. If you are interested in more than 2, why not go with 4? Or 5, 6 or 7? I have a 3 piece that I love, but I would really like to be able to subassemble into 2 halves sometimes, but you just can't do it conveniently with 3. I'd like it much better as a 4 piece rod.