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