One option for a 14-year-old to make $100 in less than 3 days is to offer dog walking services. Many people don't have the time to regularly walk their pets, and would be willing to pay someone to do it for them. As a 14-year-old, you could reach out to your neighbors, classmates, or post flyers around your community advertising your dog walking services. Depending on your local market, you could potentially make $10-20 per dog walk, and by walking multiple dogs at once or doing multiple walks per day, you could quickly earn $100 or more in just a few days.
Another idea is to organize a neighborhood car wash. Gather a few friends to help, and set up a car wash station with buckets, sponges, car wash soap, and access to a water source. Make signs advertising your car wash and hold them up to passing drivers. Charge a fair price, like $5-10 per car, and you could easily make $100 or more in a single day with enough customers. Just be sure to get any necessary permits or permissions from your local authorities before setting up.