如何使用sed命令替换文件中第三次出现的字符串。
例子：
仅更改文件中第三次出现的isto us。
我的输入文件包含：
hai this is linux.
hai this is unix.
hai this is mac.
hai this is unchanged.
预期的输出是：
hai this is linux.
hai thus is unix.
hai this us mac.
hai this is unchanged.