Since AT-700s (3U) are very head heavy, I wonder if there is a lot of people using it? That might be a reason why 3U are not common find since most of AT-700s I have seen so far are all 4Us.
I've seen one. It is very head-heavy, so much that it feels like it is heavier than my 2U Ti-10 which is also a head-heavy racquet.