The factors of 36 are: 1, 2, 3, 4, 6, 9, 12, 18, 36.
The factors of 54 are: 1, 2, 3, 6, 9, 18, 27, 54
The factors of 90 are: 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90
Therefore, GCD (36, 54, 90) = 18