I have seen many threads in the Support and Suggestions forum about this. As I have said and other people have said there are not enough games to make a multiplayer section.
there are not enough games to make a multiplayer section.
^This.
AG3 will, however, have a much better tagging system, in which every multiplayer game will be tagged "multiplayer"; so when you search "multiplayer" in the search box, or even perhaps just click on the tag (depending on whether or not there's a list of tags), all the multiplayer games will come up.
In the meantime, you can find all of the multiplayer games scattered throughout this thread.