Кодировка UTF-8 не применима.

BufferedReader bufferedreader = new BufferedReader(new InputStreamReader(System.in));
        FileOutputStream fileOutputStream1 = new FileOutputStream(bufferedreader.readLine(), true);
        FileInputStream fileInputStream2 = new FileInputStream(bufferedreader.readLine());
        FileInputStream fileInputStream3 = new FileInputStream(bufferedreader.readLine());

        while (fileInputStream2.available() > 0) {
            fileOutputStream1.write(fileInputStream2.read());
        }
        while (fileInputStream3.available() > 0) {
            fileOutputStream1.write(fileInputStream3.read());
        }