OK, Here is what each is:
CC3 or Campaign Cartographer is the MAIN ENGINE and is specifically designed to make OVERLAND maps. This is what it has the largest symbol catalog for.
DD3 is it's DUNGEON making add-on. This is specifically geared towards making Dungeons, Temples, Castles, etc. Think either INDOOR maps or outdoor BATTLEMAPS.
The CD3 or CITY DESIGNER is obviously geared towards making cities, towns, forts, etc.
Now, even if you don;t get something like DD3 (or even if you do), think about getting the SYMBOL PACKS. These give more symbol options for making your maps.
Bear in mind, just because you don;t have DD3 does not mean you CANNOT make a Dungeon map, it simply means you are lacking some of the tools (i.e. You can still Build a House if you don't have circular saw, it will just be a little harder )