in which to do sth, 为带关系词的不定式作定语。为正式用法。这是定语从句的一种简略形式，还原为定语从句则为 in which he could do sth. which指代先行词three years.

He had three years in which to establish himself as prime minister. 正式

= He had three years in which he could establish himself as prime minister. 正式

= He had three years which he could establish himself as prime minister in. 较正式

省略which，再省略主语和情态动词，不定式改为带to, 得到：

He had three years to establish himself as prime minister in . 较正式

由于时间名词可以用关系副词that引导定语从句，而that经常被省略。关系副词引导的that引导的定语从句都属于非正式语体。

He had three years that he can establish himself as prime minister. 非正式。

省略that，再省略主语和情态动词，不定式还原to, 得到：

He had three years to establish himself as prime minister. 普通