That isn't possible. Each of them has infinitely many multiples. If you want the common multiples, use the usual method to get the LEAST common multiple, then multiply that by 2, by 3, by 4, etc. to get additional multiples. But here, too, there are infinitely many of them.
The multiples of 7 are 7, 14, 21, 28, 35, 42, etc.The multiples of 14 are 14, 28, 42, etc.The multiples of 21 are 21, 42, etc.The least common multiple (LCM) of 7, 14, and 21 is 42
14 21
The first 3 multiples of 7 are: 7, 14, and 21.The first 3 multiples of 14 are: 14, 28, and 42.
7, 14 and 21.
14, 21, 28.
The multiples of 9, 14 and 21 are infinite, starting with: 126, 252, 378, 504, 630, 756, 882 +126 . . .
The common multiples of 7, 14, and 21 are the infinite set of multiples of their least common multiple, which is 42. So the common multiplesof 7, 14, and 21 are 42, 84, 126, 168, and so on.Any multiple of 42.
7, 14, 21, 28, 35, 42, 49 are multiples of 7.
It is 210.
7, 14, 21 and so on.
126
7, 14, 21, 28