The least fraction that must be added is 1/18 as the LCM of the fractions is 18 and when converted into improper fractions you will get 4/3,3/2,10/9 and when you turn the denominators to 18 you will get 24/18,27,18,20/18 and when you add these three fractions you will get 71/18 and to make the numerator divisible by 18 to make it an integer you need to add 1/18 thus making it 72/18 which is 4.