Chickens will share a nest. It's good to have more than one so they can choose, but chances are they will have a favorite and all use the same one. They will not start laying eggs until they are at least 4 to 6 months old, though. What do you mean when you say you found her in the "st"?
They do need a much bigger box right away. You need to raise the heat lamp or they will get too hot and may get sick and die. Do you have a thermometer to measure the heat on the floor under the lamp? It should be about 90°F the first week. Then lower it 5° a week until it reaches 70-75°, then they won't need it any more during the day. They need a big enough box to be able to get away from the heat if they want to.