这是三只粉笔。翻译为英语。

These are three chalks.

These are three sticks of chalk.
好像chalk是不可数名词吧！不能直接说three chalks,需要用一个量词。字典上写的一枝粉笔是a stick of chalk,那三枝就是three sticks of chalk：）

These are three chalk.

these are three chalks

these are three pieces chalks

these are three pieces of chalk