Didn't know C2 had 3dbox.
In CC 3dbox doesn't do collisions, so you will have to use a dummy.
Even then collisions cant be detected on the z plane, so you will have to use a work around. One way is to add the dummy in a container to the 3d box. Then in events always have the dummy set position to the 3dbox.
If you have multiples I would suggest using sprite's picking conditions. Something like:
->distance(dummysprite.x,dummysprite.y,mousex,mousey) < n amount
-->dummysprite pick closest to mousex,mousey- do stuff