读取不带数组或字符串的字符系列

i am struggling with one problem in my program. For input I am given ID that is series of 3 upper case letters, like ABC. The problem is I have to read them somehow without using any other header file than <iostream> and I cannot use string nor any array types, such as char[] or string[]. How can I read series of multiple letters? Can I split them somehow into three chars?

稍后需要使用该ID来实现列表-这三个字母是特定列表的ID，并且还会有其他列表，并且该程序基本上必须对其执行一些操作，例如添加元素，合并等。

感谢您的帮助！