On each player's turn, the player removes one or more stones from the piles. However, if the player takes stones from more than one pile, the same number of stunes must be removed from each of the selected piles.
For example, (0,0,13), (0,11,11) and (5,5,5) are winning configurations because the first player can immediately remove all stones.
A losing configuration is one where the second player can force a win, no matter what the first player does.
For example, (0,1,2) and (1,3,3) are losing configurations: any legal move leaves a winning configuration for the second player.
Consider all losing configurations ($x_i$,$y_i$,$z_i$) where $x_i ≤ y_i ≤ z_i ≤ 100$. We can verify that $\sum (x_i + y_i + z_i) = 173\\,895$ for these.
Find $\sum (x_i + y_i + z_i)$ where ($x_i$,$y_i$,$z_i$) ranges over the losing configurations with $x_i ≤ y_i ≤ z_i ≤ 1000$.