三者表示的意义差不多,都表完成意义．
以下为本人认为的细微差别．
already 表”已经”,一般用在助动词后,行为动词前．
e.g.They have already left their motherland.
他们已经离开了祖国．
just表”刚刚”,一般也用在助动词后,行为动词前．
e.g.They have just left their motherland.
他们刚离开他们的祖国．
yet表”已经”,用在否定句中,一般在句末,有时用于助动词后．
e.g.They haven't left their motherland yet.
or They haven't yet left their motherland.
他们还未离开他们的祖国