Just - I just did that. That just happened. I just want to eat something. Refers to time in the present.
Yet - We have yet to finish it. We haven't finished yet. It's not 4pm yet. Yet, I have to go. An inferred way of refering to the time.
Already - We already finished. We finished already. It is already 4pm. You have to go already? - Refers to the present time. Can express impatience.