How do you find the greatest common factor and least common multiple with three numbers?

Same as with two.

Example: 30, 42, 54

Factor them.

2 x 3 x 5 = 30

2 x 3 x 7 = 42

2 x 3 x 3 x 3 = 54

Select the common factors.

2 x 3 = 6, the GCF