You have some unknown integer

, and you know that adding this and the next two integers,

and

, gives a total of 57.
This means

The task is to find all three unknown integers. Notice that if you know the value of

, then you pretty much know the value of the other three integers.
To find

, solve the equation above:

So if 18 is the first integer, then others must be 19 and 20.