To find which number is divisible by 3:
Add each digits of each number
126 -> 1 + 2 + 6 = 9 (9 is divisible by 3 so 126 is also divisible by 3)
234 -> 2 + 3 + 4 = 9 (9 is divisible by 3 so 234 is also divisible by 3)
21 -> 2 + 1 = 3 (3 is divisible by 3 so 21 is also divisible by 3)
125-> 1 + 2 + 5 = 8 (8 is NOT divisible by 3 so 125 is NOT divisible by three)