It's only a flesh wound

$\large \square\square\square \ + \ \square\square\square \ + \ \square\square\square$

You are given that the numbers $1,2,3,\ldots,9$ are to be filled in the square boxes as shown above (without repetition) such it is a sum of three 3-digit integers.

Of all $9!=362880$ possible arrangements, let $M$ and $m$ denote the maximum and minimum value of the resultant number. Find the value of $M-m$ .