Having played the first two, I'd say that so far my impressions (about 6 or so hours in) of AT3 are that it's the weakest of the lot - both in terms of gameplay and characters. For all of their faults, AT1 and AT2 had some pretty inventive battle engines, while AT3 has a watered down and unresponsive feeling tales-esque engine that just doesn't feel very fun. The characters and plot too seem very lighthearted, and I don't see how AT3 can compare to some of the stingers that the first games laid on you as the plot progressed.
Still, it's not like it's awful. The music is catchy, there's a lot of crafting and conversing to do, and the shameless exploitation is in full effect. It just doesn't feel as strong as either of the first two games.
Aside: Ar Tonelico II, despite having one of the most awful localizations ever and the previously mentioned game-breaking bugs in one of the final bosses, was still one of the most fun and entertaining RPGs I've played in years.