If you wanted to create an opening door for example, select the polys for the door and in the rollout on the right, hit the 'Detach' button and from the popup, select 'As Object'. Now the door is a completely separate mesh for you to do with as you like.
For best results after that, I normally then select the 'Pivot' tab from the top-right of 3DS and adjust the pivot point for the door to where the hinges would be, you can also set a min / max angle IIRC so that you can make the door open and close to specific angles.. much easier then to control when you open / close the doors with the rotate tool for various scenes
Regards,
Ian