as I’ve remarked several times before, java3d is horrible for something you want people to actually play. It’s pile of user-unfriendly buggy non-backwards-compatible untested incompatible yuckiness.
Although, normally, I’m much gentler than that. And, as Herk says, with webstart you should be OK - just so long as the author is VERY careful to choose a specific version of j3d that actually works with his particular game.