LCM of 21 & 49 is 147, so all multiples of 147 are common multiples of 21 & 49.
Two common multiples of 21 and 15 are 105 and 315.
I think you mean some common multiples. Since 17 is prime, there are no common factors of 17 and 21. Now the first common multiple will be 21x17. You can use more 21s or more 17s to find additional multiples. Here is one 17x21x21 Here is another 17x17x21
The first 5 multiples of 21 are: 21, 42, 63, 84, and 105.
Common multiples include 168, 336 and 504
If that's 9, 14 and 21, the first three common multiples are 126, 252 and 378.
21, 42 and 63
21
7, 14, 21
63, 126, 189
42, 84, 126
63, 126, 189
84, 168, 252
126, 252, 378
63, 126, 189
126, 252, 378 and so on.
Any multiple of 21.