Despite being the overall best and most polished of the first three Seiken Densetsu games, Seiken 3 is the one we never got to see in the English speaking world. However, thanks to the wonders of emulation, and Neil Corlett's fan translation in 2000, it's now been pretty widely played.
The game is an improvement over Secret of Mana in nearly every area. I'm usually not one to go sheerly for the superficial, but this game really is worth playing just for it's graphics and music alone. I had nothing against Secret of Mana's pastel tones, but there's a greater range of color used here as well as more detail in the sprites and background graphics, and the whole thing really just looks wonderful. Add in some nice parallax background layers and it's a joy just to explore the different areas and see what they have to offer. Hiroki Kikuta also returns on the musical front, and has another score in a style similar to that of the previous game. It's just as good, if not better.
Gameplay is similar, yet there are some major differences. There are now six playable characters, of whom you can choose three as your team. While the overall story remains mostly the same regardless of who you choose, your choice of main character determines where you start out and how the events of the story progress, as well as framing the narrative from a unique point of view. Different combinations of characters will also yield different scenes, so there's actually quite a bit of replay potential with this one.
The new battle system has not necessarily improved from that of Secret of Mana, however. First of all, three people cannot play simultaneously (though there is now a ROM patch, linked below, that makes it possible), only two in this one. You also do not get a variety of weapons as you do in Mana - each character has one fixed weapon type. But since they all do a very similar short-range attack, it really doesn't feel like there's much difference between them when you fight. The only major difference is in the spells that they learn and their base statistics.
Characters also do not "learn by doing" as they did in Mana, but have a more standard EXP system where you choose to upgrade one particular statistic when you level up. At level 18 and level 38, you have the option of changing character classes, which beefs your stats up and gives you some new spells (also swaps your color palette, usually to something wierd). With two classes available per change, which then leads to two different possible classes in the next change, this also contributes a bit to the game's replayability.
I wish I could say that beyond that, it's all sunshine and roses, but it really isn't. The game apparently had a rough beta test period, with lots of bugs creeping up, and not everything was fully resolved before the game shipped. For example, during boss battles or fights against lots of enemies, characters will sometimes inexplicably not be allowed to access their spells or item rings (while nothing else of any import is going on). The game also seems to have some issues processing multiple spells that are cast simultaneously or very soon after each other, and will sometimes do things like disregarding one entirely or letting the enemy chain two or three powerful spells one after another with no chance for you to recover in between (this game's answer to Mana's "combos of death", I guess). There's also a bug that causes agility not to actually factor into ability to evade and land hits (it's supposed to, it just doesn't), making it a nearly useless stat except for disarming chest traps. There's also numerous minor little bugs that it would be tedious to document, but that you notice here and there.
Also, when you get 6 or 7 hours in to the game, you begin to realize that there's never going to be a whole lot more to it than plowing through screen after screen of monsters while mashing the B button, only stopping occasionaly to heal or cast a spell. Each new area brings new monsters, but the fights are always pretty much exactly the same. The only thing that breaks the tedium of the pattern are the boss battles, which are often screen-filling and visually impressive, and the class upgrades which give you new "charge attacks" that essentialy are a free spell cast. I'm sure die-hards of the game will argue that replaying to combine the various classes and see what strategies they can use to complement each other is the crux of the whole game, but seriously, that's a pretty limited group of people that are going to want to devote that much time to it.
On the plus side, the story is better, or at least more coherent than the previous entries in the series. Neil Cortlett and his team deserve major kudos for this in the English version, as not only was it a major technical feat to translate the game, but they also did a great job with localizing the dialogue (better than a number of Square's "professional" efforts). Anyway, the game's plot isn't Pulitzer material or anything, but it is markedly less vague and headache-inducing than all the other efforts in the Mana series.
The game can be repetitive - it really could have done with some side quests, or other things to do in the dungeons besides plow through screen after screen of combat while taking a mostly linear path to the boss. However, it's also lovely and often fun, especially if you have multiple players, and it's worth the journey for the visuals and music alone. I think on the whole it's the best game in the Mana/Densetsu series, even though I felt the battle system here was a bit weaker than that of the previous game.
* Translation Patches
* Three player patch
* Gameplay Video