Hi Frank,
In 6.x/7.x, we do not format files and assign them to the DB. We just give it directories and DB2 adds 512MB files as needed to those directories. That's why the directories have to be on different filesystems, otherwise it does not give you more space, just more directories. If the 3 directories you listed are 3 different filesystems, then yes you can extend the DB online.
The only thing that needs to be done offline is if you want to manually redistribute the database evenly across all directories after adding the space, as opposed to let it occur naturally which can take several weeks or more. You will not take a performance hit by not redistributing it, but you will not see the performance gain immediately either.
Marc...