How to compute acceleration from accelerometer raw data
As far as I understand, you will get the acceleration in [g] on each axis by dividing each of the 3 values you obtained by 256 (because your sensitivity is 256 counts/g).

To avoid any problems with the division of integer numbers, I suggest you to do in this order, for each value:


Convert the number into a signed long, to avoid any overflow in the future
Multiply the value by 1000
Divide the value by 256: you will then obtain the acceleration value in milli-g, rounded to the lowest mg.


Tip: if you want to round it to the nearest mg (instead of the lowest), you can add 128 (half the number of count / mg) before you divide the value, but after you multiplied it by 1000. 

Tip 2: as you mentioned in your comment, you can use a bitshift of 8 instead of the division by 256 to improve the efficiency of the calculation (there is a chance that your compiler would optimise the code by automatically doing it anyway)