For gorgonians, you can get away with dry feeding the for that period (FM foods) of time but for suns and dendros, I wouldn't try it. Both would need meatier types of foods (ie: mysis shrimp) and that is something that isn't easy setting up a refrigerated autofeeder for 3 weeks. If you can, have someone stop by 2-3 times a week to feed. Also I wouldn't think of letting my Azoox tank go 3 weeks without a water change.
Mike