The first three Dragon Warrior games came out in rapid succession in Japan in the late 1980s; their Western localizations were parceled out more slowly, and also updated with a smoother-to-play and better-looking game engine. As such, the first 3 games of the series were still sort of feeling their way along to the ultimate formula that the rest of the series would stick with. Dragon Warrior 3 is perhaps the most experimental of the whole series, as it fuses in the Western dungeon crawl style of giving the player an assortment of character classes to pick from and letting them create their own stable of characters at the outset whom you can switch between at any time. It's unique in that it's the only game of the series to do this; the rest of the mainline series would saddle the player with mandatory story-based characters, but allow them to change character classes to varying extents.
Unique side-view intro animation. You'll never see anything like this again for the rest of the game.
This one also still has the relative lack of storytelling present in the first two games. Some Archfiend is looming large and threatening to destroy the world just because That How He Do, though he never really seems to take any proactive steps to make this happen. Meanwhile, the leaders of the free world decide the best response to this vague threat is to send a bunch of untrained kids after him without providing them any material aid, forcing them to pay retail for all their equipment, and bogging them down in time-wasting sidequests any time they need something for their quest. With a stable of supporting characters that you create entirely out of the blue, don't expect any improvements in the way of character development either.
The presentation has improved just a bit, though. It's still the same clunky tile-based engine as the previous games with the same battle engine and backgrounds as DWII, but the world is a bit better constructed and more elaborate with better use of color. DWIII also adds a day-night cycle; different enemy types appear in the overworld at night, and different stuff is going on in the towns. By this point composer Koichi Sugiyama also had full mastery of the NES sound chip, and chiptunes aficionadoes will find the soundtrack he wrings out of it here is almost worth the price of admission by itself.
It's also a bit more feature-rich. There's now a bank that allows you to stash loot and circumvent the "lose half your gold when you die" rule (to some degree.) You'll have to return to the starting town for this, however, as well as for the Inn where you create and swap out characters and stash unwanted items. You have an active party of your Hero plus three now, and can stash up to four more created characters at the stable. You pick gender plus class - Soldier, Fighter, Mage, Pilgrim, Goof-Off or Merchant - and in a nice touch for 1980s NES, each gender/class combination gets its own unique sprite. A little past halfway through the game you'll also encounter a Shrine (a la Final Fantasy) that allows you to switch character classes. It functions a bit differently than Final Fantasy, however - there's only one "upgrade" class, the Sage, and you have to find a special item to do it (the otherwise useless Goof-Offs being the exception, as their only positive quality is being able to upgrade to Sage without the item when they hit level 20.) Switching character classes is instead a means of making broken hybrids, such as Soldiers with healing abilities or Mages with the ability to use the best armor. You pay a hefty price, though - characters have to be at least level 20 to change classes, and when they do their level will be reset to 1 and their stats halved (though they retain all spells learned.) The game makes you put in some serious grind time if you want your broke overpowered characters; many players will find it optimal to simply stick with the starting class all the way to the end and never change, except perhaps upgrading a Pilgrim to Sage. The Hero doesn't get the benefit of all this Class Changery, either, though he's a well-rounded chap who can use all the armor and weapons and also gets a range of both healing and attack spells.
The result of allowing you to have a stable of multiple character classes (and halving levels periodically) meant some rebalancing of the traditional engine. You'll find Dragon Warrior III a little more on the lenient side as far as difficulty goes, especially if your most recent memory of the series coming in is the ridiculous endgame of Dragon Warrior II. The Hero gets the Return spell fairly early on, making travel about the world map much more easy and convenient. Since you have a party of four, enemy groups can run up to five now, but they only tend to be overwhelmingly difficult when you hit the occasional mobs that can spam a sleep spell every turn. Inventory juggling is also no longer the major annoyance it was in the first two games, as with four party members you'll usually find a comfortable amount of space, and can stash items that aren't presently useful in the vault back at the starting town.
That's not to say the game is a complete cakewalk. It still pulls the old "slot machine" bullshit in the later reaches, with a bunch of enemies that spam freeze and instant death spells in the final few areas, and a particularly irritating Limbo spell that tosses one character back to the starting town. The Limbo enemies are prevalent in the final dungeon, and if they manage to land the spell it essentially forces you to bail on the dungeon and go recover the character. The final 1/10 of the game falls more under "mild to moderate annoyance" than the insanely brutal difficulty jag seen in the lattermost reaches of the previous game, though, and the final boss is much less of a bear to tackle here.
At the time it was released, the central appeal of Dragon Warrior 3 was that it worked in some pioneering ideas, at least relative to the console JRPG genre to that point - the day/night cycle, gambling on monster battles, customized characters with class changing, etc. 20 years later, the implementation of those ideas just looks primitive and kind of clumsy. Dragon Warrior 3 is still solid, however; I found it entertaining enough to stick with to the end. It has its little annoyances, but overall did a much better job of difficulty balance and dungeon design than the previous game did.