Hello!
To solve this you have to find what the biggest number that each number can be divided by
You can do this by listing the multiples of the smallest number
The multiples of 6 are
1, 2, 3, 6
Now you look and see what the biggest number 18 and 15 can be divided by evenly
15 can't be divided by 6 so it is not 6
All three numbers can be divided by 3 so this is the answer
The answer is 3
Hope this helps!