To find all common multiples of numbers, you need to list all multiples of the numbers up to the maximum value:
3: 3,6,9,12,15,18,21,24,27,30,33,36,39,42,45,48,51,54,57,60,63,66,69
7: 7, 14, 21, 28, 35, 42 , 49, 56, 63, 70
All of the COMMON multiples of 7 and 3 are 21, 42, and 63