This secret number is a 3 digit number. the digits are in counting order. if you add the digits, the sum is 12. what is my number?

1st number : x

2nd number : x + 1

3rd number : x + 2

x + x + 1 + x + 2 = 12

3x + 3 = 12

3x = 9

x = 3

1st number : x = 3

2nd number : x + 1 = 3 + 1 = 4

3rd number : x + 2 = 3 + 2 = 5

The number is 345.

Check:

x + x + 1 + x + 2 = 12

3 + 3 + 1 + 3 + 2 = 12

12 = 12

3 + 4 + 5 = 12