Find the prime factorization of each value.
21 is 7*3
30 is 3*10 which can be further broken down into 3*2*5
44 is 4*11 which can be further broken down into 2*2*11
What do they share in common?
21 and 30 each have a 3 in common.
Therefore the GCF(21, 30) = 3.
21 and 44 share a 2 in common.
But if we're looking at all three of them together, they share nothing in common.