hope I understand the problem right.
You can see which camera is currently active in some sprite:
this returns the name of the active camera camera(which you
are viewing trough).
if you know the name of the camera you can do this to get the
index(Ithink there is a easier way..):
w3d" is your 3dshockwave
repeat with id= 1 to w3d.camera.count
if w3d.camera[id].name = "cameraYouSearchFor" then put id
-- the returned 'id' is the index of the camera with the name