At this point, we have some absolutely impressive Portal 2 mods that, in my opinion, definitely make up for not having Portal 3. We’ve got Tag (the paint gun mod), Thinking With Portals (a short one that gives you the ability to create a clone thing that copies recorded actions), and I’ve been playing through one I just found out about called Revolution that is pretty cool so far.
I personally like to think that the fans have done a good enough job at creating mods and maps that a Portal 3 would be redundant.