DD3 adds a lot of functinality as well as SYMBOLS, templates etc.

The Symbol catalog that comes with CC3 for Dungeons is extremely limited, consisting of only 22 symbols: a bed, a chair, a chest, a coffin, a rocky column, a door, a glyph, a skeleton, a pentagram, a pillar, a 10' pit (closed), a potted plant, a large circular pool, a pile of rocks, a torch in a sconce, a spiral stair, a straight stair, a table with chairs, 2 trap doors, and a pile of treasure. Kinda basic.

DD3 on the otherhand comes with 14 different symbol catalogs: Cave Features; Containers/Treasure; Debris; furniture; Doors & Windows; Temple Furnishings; Pits/Traps; Stairs & Ladders; weapons; monsters; simple; geomorphs; and walls.

Each of those catalogs come with 20+ symbols PER CATALOG.

Again, all the functionality is in the basic program (CC3) and in fact DD3 & CD3 will NOT run without CC3, but as a user of the Suite I can honestly say, they are ALL worth it. If you can afford it, I would suggest at the LEAST the WORLD BUILDER (Fractal Terrains Is an Awesome tool for starting a world), and that gets you FT, CC3, DD3, and CD3. But the best BANG fer your buck is definitely the follwoing:

The Fantasy Mapper
$218.50 / £148 = save 12.5%.

Everything you need to bring your fantasy campaigns to life. With this bundle you can create rich maps of any area, from complete worlds, through countries, cities and villages, down to individual floorplans and skirmish sites.
Includes:

Campaign Cartographer 3, City Designer 3, Dungeon Designer 3, Character Artist Pro, Dioramas Pro, Symbol Set 1 and Symbol Set 2.
Another invaluable tool is the Annual, which is a lowcost subscription for additional goodies each month (at $30, that means a new symbol set, templates, etc each and every month).

If money is tight though, get CC3 and Symbol Set 2 (Dungeon Symbols), this will give you a lot of functionality as well as some good symbol sets and get you on your way.