It is in these volumes that the anime and the manga start to diverge, with the differences being more issues of timing than radical changes in the story. We are introduced to Chika Ogiue, and while we don't quite get the fact that she is into yaoi (a sin that many in the US gleefully indulge in) she starts out very bitter but is brought short by Kasukabe of all people, who points out that Ogiue needs these people, these otaku, for companionship if for nothing else. Also in the manga you can see the weird trajectory of Kasukabe's and Madrame's non-relationship a little better, along with Kasukabe's journey if not to otakudom, at least to her acceptance of it. Sasahara starts growing into a leadership role by helping the Genshiken move out of their apathy and into action. This is a strong continuation of the series, with no major missteps that shows that the characters can remain true to themselves yet still grow.
Genshiken vol. 3 and vol. 4 are both available from Right Stuf, Intl., an online retailer specializing in anime and manga.