Common multiples are multiples that are common to two or more numbers.
9: 9, 18, 27, 36, 45, 54, 63, 72, 81, 90, 99, 108, 117, 126, 135, 144, 153, 162, 171, 180, 189
21: 21, 42, 63, 84, 105, 126, 147, 168, 189
The first 3 common multiples of 9 and 21 are 63, 126 and 189.
63, 126, 189.
If that's 9, 14 and 21, the first three common multiples are 126, 252 and 378.
21, 42 and 63
21
42, 84, 126
63, 126, 189
84, 168, 252
63, 126, 189
7, 14, 21
63, 126, 189
126, 252, 378
126, 252, 378 and so on.
Any multiple of 21.