Take your time driving into Cataloochee, because the road engineer's philosophy was one good turn deserves another. There is not a lot of room for error on the gravel portion of the road. If you're planning on going in three weeks, keep an eye on the weather. I hate to even mention the word snow, but it is not out of the possibility that time of the year. I camped there during the last week of April a few years ago in the snow. Cool or even cold nights (as in the 20's) are highly possible.
Bear, elk, and a few boar are in Cataloochee. The bear won't bother you if you don't bother them or inadvertently invite them over for snacks. Hang your food supply. Snakes shouldn't be an issue that early in the year. However timber rattlers and copperheads are present. As for planning on fishing all the tributaries in Cataloochee, you'll never do it in 3-4 days. You should have a large time on your trip, it's one of my favorite locations in the park.