3 way merge sort in C

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

// Merge three sorted subarrays into one sorted array
void merge(int arr[], int left[], int middle[], int right[], int n1, int n2, int n3) {
    int i = 0, j = 0, k = 0, l = 0;

    while (i < n1 && j < n2 && k < n3) {
        if (left[i] <= middle[j] && left[i] <= right[k]) {
            arr[l++] = left[i++];
        } else if (middle[j] <= left[i] && middle[j] <= right[k]) {
            arr[l++] = middle[j++];
        } else {
            arr[l++] = right[k++];
        }
    }

    while (i < n1) {
        arr[l++] = left[i++];
    }

    while (j < n2) {
        arr[l++] = middle[j++];
    }

    while (k < n3) {
        arr[l++] = right[k++];
    }
}

// Recursive three-way merge sort function
void mergeSort(int arr[], int n) {
    if (n > 1) {
        int n1 = n / 3;
        int n2 = (2 * n) / 3;
        int n3 = n - n1 - n2;

        int left[n1];
        int middle[n2];
        int right[n3];

        for (int i = 0; i < n1; i++) {
            left[i] = arr[i];
        }

        for (int i = 0; i < n2; i++) {
            middle[i] = arr[i + n1];
        }

        for (int i = 0; i < n3; i++) {
            right[i] = arr[i + n1 + n2];
        }

        mergeSort(left, n1);
        mergeSort(middle, n2);
        mergeSort(right, n3);

        merge(arr, left, middle, right, n1, n2, n3);
    }
}

int main() {
    int arr[] = {12, 11, 13, 5, 6, 7, 3, 8, 10, 9};
    int n = sizeof(arr) / sizeof(arr[0]);

    printf("Original array: ");
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    mergeSort(arr, n);

    printf("Sorted array: ");
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    return 0;
}