Detect multiple 3d shapes in an image

I'm currently learning OpenCV for a project I recently started in, and need to detect 3D boxes (imagine the big plastic boxes maybe 3ft x 2ft x 2ft) in an image. I've used the inRange method to create an image which just had the boxes I'd like to detect in it, but I'm not sure where to go from there. I'd like to get a 3D representation of these boxes back from OpenCV, but I can't figure out how. I've found quite a few tutorials explaining how to do this with just one object (which I have done successfully), but I don't know how I would make this work with multiple boxes in one image. Thanks!