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.)