#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main() {
	char s[105];
	gets(s);
	int len = strlen(s);
	for (int i = 0; i < len; i++) {
		if (s[i] >= 'a' && s[i] <= 'z') {
			s[i] += 3;
			if (s[i] > 'z')s[i] -= 26;
		}
		else if (s[i] >= 'A' && s[i] <= 'Z') {
			s[i] += 3;
			if (s[i] > 'Z') s[i] -= 26;
		}
		else {
			continue;
		}
	}
	puts(s);//输出字符串

	return 0;
}

第二种

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main() {

	char s[105];
	gets(s);//为了不让空格干扰而不使用scanf
	int len = strlen(s);
	for (int i = 0; i < len; i++) {
		if (s[i] >= 'A' && s[i] <= 'Z') {
			s[i] = 'A'+(s[i] - 'A' + 3)%26;//先算加三后看是否超出，超出取余
		} 
		if (s[i] >= 'a' && s[i] <= 'z') {
			s[i] = 'a' + (s[i] - 'a' + 3) % 26;
		}
	}
	puts(s);

	return 0;
}