Let

x = the first number

y = the second number

z = the third number

The sum of three numbers is 55:

x + y + z = 55

The third number is 2 times the second:

z = 2y

The first number is 9 less than the second:

x = y - 9

We have the following system:

x + y + z = 55

z = 2y

x = y - 9

........

........

x = 7

y = 16

z = 32

The numbers are 7, 16 and 32.