252, 504, 756 and so on.
252
Any multiple of 252.
No, they are factors.
126, 252, 378
84, 168, 252
Any multiple of 252
252
No, they are factors.
If that's 9, 14 and 21, the first three common multiples are 126, 252 and 378.
2. Nothing else will work
84, 168, 252 and so on.
84, 168, 252