Doesn't have to be a separate file. But it has to be a separate group with its own name.
Each part that can move must have its own group name.

The figure itself would have "hull" as the root actor, comparable with "hip" for a human figure.

The canopy is directly attached to the hull and has no moving subparts, so it can be one group "canopy"

The nose wheel is different. It consists of a leg that can be retracted into the hull, and it has a wheel that can turn. The "leg" consists of two parts, the top part can't turn around the vertical axis, while the bottom part can, to allow for steering.
Further, the leg will be connected to a piston system for the retracting. The piston system consists of two parts.
Each of these parts must be a separately named group.

Export to .OBJ is preferable, it's Poser's native format. 3DS may not support more than 32,000 polys (depends on the exporter implementation) and may not support groups.

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