Hi Gordywill, is there anything you want to do in particular? If there's something that's missing or wrong or doesn't have the correct variant, that usually spurs folks on to do something about it. For me it was the lack of 'authentic' airfields in cfs3 and that's become a hobby in its own right. Having a focus will help!
The aircraft modelling and terrain SDKs are very useful - see Major Magees link above.
Very, very briefly.....
.dds - these are the image files used by CFS3. Editable with Paint Shop / Photshop / Paint dot Net / Gimp to mention a few.
.mos - Used with the above, enables damage and markings etc. to be added to aircraft and ground textures for airfields, factories etc.
.xml - A main file type used by cfs3 for all manner of things. Editable with Notepad or word.
.xdp - Used in models and defines type, country, weapons, damage viewpoints etc. Also for runway definitions
.cfg - Flight models, editable in Notepad or word
.air - Flight models, editable with 'Air-ed' or similar, downloadable from lots of sites.
.csv - Defines ground objects in the cfs3 world, Docks, airfields, factories etc. Editable with Excel (best option) or Notepad.
.Lib - CFS3 compiled version of the above, created with tools in the SDK
.bdp - a cfs3 generated file, not readily editable but can be deleted.
Plus some others that I've forgotten
Cheers, Clive