126
7: 7, 14,21,28,34,41,49,56, 63,70,77,84, 91, 98, 105, 112, 119, 126
18: 18,36, 54, 72, 90, 108, 126
21: 21, 42, 63, 84, 105, 126
you can see that when you list out the multiples the least common multiple is 126 because it's what they all have in common