Furthermore, what would you be trying to accomplish by doing a 3 way? Sure you could use bigger woofers to get more lowend extension but the box would be huge(r). I think most use a sub when wanting to extend the lowend of the response.

On the other hand integrating subs into an enclosure (1 or 2 per side below the array) is common and has been done. No real design challenges here, but you do need to hope that the ideal placement for the subs is the same as it is for your mains.

Do a search for "Mcintosh array" for more info on a 3 wayish design and look for Rick Craig's opinion of it.

HTH

Later,
Aaron