Translation of "be useful" into Japanese

We have three spare rooms, none of which can be used.

空き部屋が3つあるが、そのうちのどれも使えない。

Read such books as will be useful some day.

いつの日か役に立つような本を読みなさい。

This is a map which will be useful when traveling by car.

これは車で旅行するのに役立つ地図だ。