int i, j, k;   int cube[N][N][N];

// for each element (matrix)
for (i = 0; i < N; i++) {
    // for each row of current matrix
    for (j = 0; j < N; j++) {
        // for each element in current row
        for (k = 0; k < N; k++) {
            // process cube[i][j][k] (an integer)
        }
    }
}