Been having a think about this and I reckon it can be done.
I think you're on the right track with the 3 separate models since I don't think you can animate the same part twice :-\
I reckon it needs to be done in steps with a script(s) to link them together
Step 1 - Tank carrying bridge drives into position, "launch bridge" option in action menu activated
Step 2 - Stowed bridge to extended bridge using a rotation animation with the rotation point far enough away that it looks like it's sliding rather than rotating (like the doors on the SEB Huey)
Step 3 - Script called to swap model to extended bridge model
Step 4 - Script activates "lay bridge" option, animation moves the bridge forward towards the ground
Step 5 - Script swaps model for tank without bridge model and adds model of bridge in front of it which can be driven over.
The bridge can be picked up by running another script which runs the above in reverse.
I know nothing about scripting, so if someone who does could give some idea of the feasability of this it would be much appreciated.
As for animations try Colonel Klinks tutorials (
http://www.dc3d.co.nz/)
Hope this makes sense cos my brain hurts now.