考虑后面三个字母加三会回到开头 

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

int main()
{
    char s[100];

    gets(s);//读取一行
    for (int i = 0; s[i] != '\0'; i++)
    {
        if (s[i]>='a'&&s[i]<='z')
        {
            s[i]=(s[i]+3-'a')%26+'a';//考虑z->a循环
        }  
        else if (s[i]>='A'&&s[i]<='Z')
        {
            s[i]=(s[i]+3-'A')%26+'A';
        }          
    }
    printf("%s",s);
    //system("pause");
    return 0;
}