Use prime factorization.
3 = 3
6 = 3 × 2
7 = 7
So the lowest common factor is 2*3*7 (drop the other 3 because it's a duplicate) which is equal to 42. Any multiple of that will be a multiple of 3, 6, and 7.
So all we need is a multiple of 42 which is between 55 and 101, which would be 84.
