There is none because the Least Common Denominator (or LCD) is the Least Common Multiple (LCM) of the denominators of two or more fractions.
84
It is: 42
84
84/14 = 6 84/7 = 12 84/3 = 28 Hence 84 is the least common denominator
7 x 6 x 5 = 210
42
126
84
84
It is: 42
420
84
42
1008
126
It is 630
The least common denominator of 6,7, and 8 is the least common multiple of those numbers. This is 168.