For calculating lenght of string ,you should use strlen function which is included in string.h library file.
#include<stdio.h>
#include<string.h>
int main()
{
char name[100];
int len;
printf("Hello, User! What name should I call you?");
scanf("%s", name);
len=strlen(name);
if (len <= 2) {
printf("Nope, Your name has to be atleast 3 characters!");
} else {
printf("Oh, Hi, %s!", name);
}
return 0;
}