So, your wanting the entities to appear between the backround 3d scene and the entities in front?
If so:
Set the cameraclsmode to clear the z-buffer, but not the colour buffer.
Render the backround scene.
Hide backround scene
Render your entities.
Hide your entities.
Render whatever else you need.
This way, what you render, won't be cleared upon the next renderworld. It basically causes the render to become printed onto the backbuffer as a bitmap image.
If so:
Set the cameraclsmode to clear the z-buffer, but not the colour buffer.
Render the backround scene.
Hide backround scene
Render your entities.
Hide your entities.
Render whatever else you need.
This way, what you render, won't be cleared upon the next renderworld. It basically causes the render to become printed onto the backbuffer as a bitmap image.