First off the last Grandia I played was the second one and I absolutely loved it. I never played this one or Xtreme, but I do want to eventually give both a try if they are worth it.
I don't really care too much if a game, in this case an RPG, is linear as long as it has a good battle system. Having a good story is a plus but for me it's not as important as having good gameplay. I've played RPGs where the story and characters didn't interest me but the battle system, game pacing, and character management held me enough to play to the end.
Also, about how many hours does it usually take to get through it minus any side quests? I personally don't like games that take over say 40 hours of your time because there's a ton of games I have yet to check out.
The main problem people have Grandia III is that the story/characters aren't very engaging, so if that's not a problem for you then go for it.
The battle system is probably my favourite out of any RPG I've played; it's basically the same system as Grandia II. I personally prefer it to Grandia II because it isn't as easy to overpower your characters/spells/skills, so the battles are consistently challenging. The characters are pretty customisable, being able to equip any spell or skill.
What the game is missing is sidequests; there aren't any bonus dungeons or anything, the game is very linear with just a couple of basic minigames.