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.

Just is used to show an action happened a very short time age as in "you just missed him" meaning he left moment age. Yet means the task has not been completed but there is time to do it. Already means the task has been completed ahead of schedule or earlier than expected. Regards, John