You can expect a major increment in the quality of the gameplay. Grandia 2 is way too easy, thus you never actually need to use the battle system to the fullest, namely doing Cancels, Evading and stuff. G3 adds an Aerial Combo option to G2's battle system, and actually offers challenging gameplay, unlike the previous entries in the series. An optimal challenge level has been achieved. Here even regular encounters can pose a threat if you take a victory for granted and start playing sloppily.
Storyline wise, a big no. Nothing here is comparable to the story/characters of G2. In fact, most players are severely annoyed by the fact that two characters that made the story hold promise leave you early on, never to be seen/mentioned again. If you are playing this game with this kind of intent, consider yourself warned. There's a cool scene here and there, but nothing quite like G2 IMO.
So, your motivation for playing this game will be the fantastic battle system, which I'd go as far as stating as one of the best turn based battle systems I ever experienced (only SMT: Lucifer's Call make me doubt about it). If what you want is watch glorified cutscenes about supposedly deep characters, and regular battles where all you do is mash X to make your characters attack and easily win (like FFX, for instance), stay away.
"You can plainly see why I'm a Boss character, and not some thug" - Kanon