is it important to have the partymembers levels balanced?
or can i like only use 3 characters for the entire game?

Not really. Its pretty easy to complete with 3 characters.

Pretty much if a party member dies you can swap them out with another person so that you don't have to raise them immediately, so it would be better to level up at least one or two extra people...and if its the international version then it is more fun b/c you have different classes. But it is possible to do it with just 3.

you can even leave their level at base if you know what you're doing

too bad the izjs version never made it to yurope