What is the Least common denominator for 6 and 7 and 9?

There is none because Least Common Denominator (or LCD) refers to the Least Common Multiple (LCM) of the denominators of two or more fractions or mixed numbers. Even if you converted your whole numbers (x) to their fraction equivalents (x/1), the LCD would always be 1.

If those are denominators, the LCD is 126.