Imagine that, my friends are talking about something. Because I know few about that topic. I want to just listening.
Which one is more idiomatic for this kind of situation?
Are there other better expressions for this situations?
Out of the three you posted, the second one is the most idiomatic.
"I am here just to listen" would be understood to mean what you want - to learn about the topic from more knowledgeable friends.
But you could specify "I am here just to listen and learn" - English expressions love alliteration (the same consonant starting consecutive words) and this would also indicate you are not eavesdropping, more looking to learn.
(As a side note, you said: "Because I know few about that topic" which is incorrect. A better expression would be "Because I know little about that topic". "Few" is for countable things, whereas knowledge is not countable.)
