- JUST: the action has been done short time ago. I've just seen Peter downstairs; he looks great.
- ALREADY: it indicates the action is finished, and before than expected. My teacher has already corrected the exams.
- YET: this shows that the action is not finished or you don't know if it's finished. I haven't seen Susan yet. Has she called you yet?
As usual, the best way it to do some exercises to practise:
I hope this will be helpful.