你的理解是对的：

误：This is my three classmates.

正：These are my three classmates.

类似例句：

These are my three children. 这些是我的三个孩子。

These are my three brothers. 这些是我的三个兄弟。

These are my five favourite pictures. 这些是我最喜欢的五张照片。

I enjoyed my three years at university. 我在大学度过了三年美好的时光。

My three years at college flew by. 我在大学的三年一晃就过去了。

Jill is the eldest of my three children. 吉尔是我三个孩子中年龄最大的。