Coop maps for more than 2 players?
I'm currently looking for coop maps that are made for preferably 3 players, maybe 4 would be fine too. I think just making such maps that require more than two players in a meaningful way is not easy, but there have to be some I'm sure. 
Since there is no filter option when browsing the download section, and putting a "3" in the forums' search box on the left is a stupid idea I apologize if there already is a thread with a list of such maps somewhere. Just couldn't find it. 
Maybe it is not possible to create a proper exit that will load the next map or anything like that, and you might need to enter a few console commands after loading the map, but just the fact that you can get 3 or more players in one map should make it possible to create puzzles that require 3 players to solve it, just from how they're structured.

It seemed to work well at a first glimpse...
Zahl wrote:
Hm, what a pity.
It seemed to work well at a first glimpse...
Not sure what glimpse you saw, but things do start to screw up with viewpoints when there are three or more sets I believe. I tried it once in singleplayer and I think if a portal from all three sets is in view the game starts overlaying the rendering of specific elements.