We use ‘just’ for things that happened very recently.
Example: I have just bought a car. ( Maybe yesterday :/. We don’t now but a short time ago )
Subject + have/has + just + Verb3
We use ‘already’ for something that happened before now. ‘Already’ tells us that there is no need for something anymore.
I don’t need money. I have already had a lot of money.
Subject + have/has + already + Verb3
We use ‘yet’ in negative sentences when we have an expectation for something to happen.
I haven’t finished homework yet.
Subject + haven’t/hasn’t + Verb3 + ….. + yet.