Enough clothes for however many days you're staying plus one extra. Or if you're wearing costumes all three days, at least two extra outfits just in case something happens.
For cosplayers: Sewing repair kit, duct tape, hot glue gun.
Make a list of everything you'll need for your costumes (hair products, straightener, dryer, props, shoes, accessories, etc) and go over it at least once.
Some type of non-perishable food. Cup 'o noodles works wonders, plus it's really inexpensive. High protein snacks that you can eat when you're in a hurry.
Comfortable shoes for when you're not in cosplay.