Filling a 3D array in a pointer with values in C

Question

I am trying to adapt this existing SO answer, for assigning values into a 3 dimensional structure. I'd like to

1. Define an int ***a3d
2. Call fill_array(int ***arr, int levels, int rows, int zIdx)
3. Print one of the values in the array

Here's an attempt based on the other SO answer:

#include <stdio.h>
#include <stdlib.h>

#define ZALLOC(item, n, type) if ((item = (type *)calloc((n), sizeof(type))) == NULL) \
                                  fatalx("Unable to allocate %d unit(s) for item\n", n)

static void fatalx(const char *str, size_t n)
{
    fprintf(stderr, "%s: %zu\n", str, n);
    exit(1);
}

static void fill_array(int ***arr, int levels, int rows, int zIdx){

  int count = 0;
  int i, j, k;
  ZALLOC(arr, levels, int **);
  for (i = 0; i < levels; i++)
    {
      int **data;
      ZALLOC(data, rows, int *);
      arr[i] = data;
      for (j = 0; j < rows; j++)
        {
          int *entries;
          ZALLOC(entries, zIdx, int);
          arr[i][j] = entries;
          for (k = 0; k < zIdx; k++)
            {
              arr[i][j][k] = count++;
            }
        }
    }
}

int main( int argc, char** argv )
{  /* dimensions of the array */
  int d1 = 800;
  int d2 = 600;
  int d3 = 3;

  /* define the 3D array */
  int ***a3d;

  /* fill it with values */
  fill_array(a3d,d1,d2,d3);

  /* print one value in the 3d array */
  printf("Example value is %i\n", a3d[5][5][1]);
}

Unfortunately, I get

$ ./a.out 
Segmentation fault (core dumped)