For this case we have that by definition the Greast Common Factor is the biggest factor that two or more numbers have in common.
So, we look for the factors of 12, 18 and 84:
12: 1,2,3,4,6,12
18: 1,2,3,6,9,18
84: 1,2,3,4,6
Thus, we can see that the GCF of 12, 18 and 84 is 6.
Answer:
GCF: 6