If you've never played these games before, it's not very hard to imagine what each looks and plays like. Each is a very typical Japanese RPG for the time it was released and you've doubtlessly played other games like them.
If you were to pick up the original Phantasy Star and begin playing it with no idea of its background except that it was a Sega game, you are very likely to assume the game was for the Sega Genesis. Well, you would be wrong. This game was original for the Sega Master System. For a game on an 8-bit system, the original Phantasy Star's graphics are extremely impressive. However, 8-bit graphics they remain and so this game is, of course, the least impressive visually of the three. Since it is nearly 10 years old, it looks just downright horrible if you compare it to current games, but that wouldn't be fair now, would it?
While the graphics are very impressive for such an old game, its sound isn't going to fool anyone. The sound definitely doesn't match the quality of the graphics. Now that's not to say that the sound isn't good, but none of the music or sound effects are as good as its contemporaries, namely the first four Dragon Warrior games and the original Final Fantasy.
While the original Phantasy Star was ahead graphically of its Nintendo counterparts, Phantasy Star 2 only just matches the visuals of its brethren. This was an early Genesis Game, and for the most part has the same level of detail in its graphics as early Super Nintendo RPGs like Final Fantasy IV, Lufia and the Fortress of Doom, and Breath of Fire. You can still see the tiles, the characters are smallish, and colors aren't as varied as they could (and eventually would) be. Phantasy Star 2 continues the trend of its audio staying one step behind its contemporaries. Again, it's not bad, but it sounds like what should have been in the original Phantasy Star.
After seeing what could be done graphically on a 16-bit system at the end of its era with games like Final Fantasy VI, Chrono Trigger, and the Donkey Kong Country games, it's really a shame to see Phantasy Star 3 fall flat on it's face graphically. It really doesn't look that much better than Phantasy Star 2. It's quite possible that you just couldn't do with the Genesis what developers managed to do with the Super Nintendo, but the fact remains that the graphics in Phantasy Star 3 just don't live up to what was being done during its time. But, if all these years later the graphics from one and two don't bother you, it's doubtful you'll find three unattractive either. The audio goes forward a little more than the graphics did from two to three, but still remains sub-par.