First, the meaning is different.
not yet
Interpretation: conj But; However.
Example:
No? Get? Up? Not yet. Don't get up yet.
2. already
Explanation: adv. already; It's already started.
Example:
Me? Already? Inform? What about you? Isn't it? Where to? Coming? Over. I told you not to come here.
3. Still
Interpretation: adv. also; Still; Still.
Example:
I am? General? For what? Mine? Height. ? But? Me? Or? Feeling? I am? More fat? Than? Me? Should? Yes.
Judging from my height, I am of medium height. But I still feel a little fat.
Second, the usage is different.
not yet
Usage: it means that something has not happened or been completed, or that something has not happened. It can also be used to ask if something happened.
2. already
Usage: Used in interrogative sentences, which means "surprise".
3. Still
Usage: it means that the original situation has not changed, or that a certain situation is still going on. It can be used in affirmative sentences, negative sentences and interrogative sentences.
Third, the emphasis is different.
not yet
Analysis: mostly used in negative sentences and interrogative sentences.
2. already
Analysis: often used in affirmative sentences.
3. Still
Analysis: What happened in the past and the current situation are still going on.