If it was a smaller cut I'd say use a coping saw but if the cut goes on for 3ft a jigsaw would be better. There is a variety of blades available for most jigsaws each of which have a number of uses like wood, metal, plastics and so on. best thing is refer to your jigsaw manual and see if it recommends a blade for curved wood cutting i know my bosch one does or look at the jigsaw manufacturers website. Something else to bare in mind is the thickness of the wood your cutting and the max cut depth for the jigsaw / blade.
if only a slight curve i'd be inclined to do it with a belt sander, palm sander, hand plane or spoke shave or even a rasp. depends how much material you need to remove. if it is only a very slight curve then probably to only way to go is by hand.
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