As long as the gosling is showing any type of respiratory distress I wouldn't put her out with the flock. She needs to be kept where she won't chill or be in contact with other birds until you are sure she is over whatever this is.
How is the gosling today?
3 weeks is a might young to be putting a gosling in with a resident flock without time for everyone to get to know each other through fencing. Is she in a brooder now inside? Especially if she is sick. She needs to be kept warm and given plenty of liquids with some electrolytes/vitamins in her...