Что не таК?
package com.javarush.task.task18.task1818;
import java.io.*;
/*
Два в одном
*/
public class Solution {
public static void main(String[] args) throws IOException {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
String file1 = reader.readLine();
String file2 = reader.readLine();
String file3 = reader.readLine();
reader.close();
reader = new BufferedReader(new InputStreamReader(new FileInputStream(file2)));
BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(file1)));
while (reader.ready()) {
writer.write(reader.readLine());
}
writer.close();
reader.close();
reader = new BufferedReader(new InputStreamReader(new FileInputStream(file3)));
writer = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(file1,true)));
while (reader.ready()) {
writer.write(reader.readLine());
}
reader.close();
writer.close();
}
}