It's usually recommended to allow at least 10 square feet per chicken in a run. This is not only for the mental health and happiness of the chickens, but it's also to make the chicken keeper's job easier in keeping the area clean enough for health and odor control. You will be amazed how much chicken manure from three chickens will build up in that tiny space in only a day.
You have this run set on pavers. Is it your intention to leave it there, or move it around your yard on grass? The later would be better. It would give the chickens something to do, and help with the manure buildup problem. A hard surface like pavers is not the best thing for a chicken's feet, too.
Be aware that without a wire apron around the edge of the run, predators could dig under the edge and get inside at your chickens. And with the small size of the run, when it rains it's probably not going to provide much protection to the chickens from rain blowing in from the sides. The chickens will get wet, and when it's cold, wet feathers lose their insulating properties and the chickens could get badly chilled.