How can I set the c variable to that if entered then it should print "hello world 3" or if not entered then only the upper two should be printed?

Question

This is a program to take the input at runtime from the user and print hello world depending on the arguments entered. The user can print 2 hello world. However the third hello world is conditional depending if the c value is entered or not.

#include <stdio.h>
#include <unistd.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>


int main(int argc, char *argv[])
{
 unsigned int a, b, c;


        a = strtoul(argv[1], 0, 0);
        b = strtoul(argv[2], 0, 0); 
        c = strtoul(argv[3], 0, 0); 

        int *p= NULL;//initialize the pointer as null.

        printf("\n first no %d \n ", a);
        printf("\n second no %d \n ", b);

        if ( c == *p)
        {
                    printf("\n third no %d \n ", c);
        }

    return 0;
}

1. Suppose I run the program as ./hello 1 2 -> This would print "hello world 1" "hello world 2".
2. And if I run the program as ./hello 1 2 3 -> This would print "hello world 1" "hello world 2" "hello world 3".
3. How can I correct my program to get the desired effect?