Lets assume:

1st number[smallest number]: X

2nd number: X+1

3rd number[largest number]: X+2

As their sum is 72 so:

X+X+1+X+2=72

Adding:

3X+3=72

shifting 3 to the other side:

3x=72-3

Subtracting:

3x=69

Dividing:

X=69/3

So:

x= 23

As X represents the smallest number the smallest number is 23

1st number[smallest number]: X

2nd number: X+1

3rd number[largest number]: X+2

As their sum is 72 so:

X+X+1+X+2=72

Adding:

3X+3=72

shifting 3 to the other side:

3x=72-3

Subtracting:

3x=69

Dividing:

X=69/3

So:

x= 23

As X represents the smallest number the smallest number is 23