Chris:
All ADS programs communicate with AutoCAD via the 'RESBUF' structure, this
is a linked list. You can read all about this in the ADS Ref manual.
Once your program has got the entity you require( ads_entget, or scan whole
drawing) you can then traverse the resbuf linked list build a string of the
3d faces in your own format ready to pass across the network.
Have a look at the ads code on page 68 of the ADS Ref Manual.
Hope this gives you a start !!
Dawson