Well, the new discus is semi-social now and will actually spend time out in front of the tank with the other two. The original two are eating well, spend most of their time hanging around in front of the tank, or at least not hiding in the corners. A couple weeks ago I could see that one was larger than the other, though they are supposed to be the same age. And last week it seemed like all of a sudden one was twice the size of the other. The runt does get the majority of the bullying in the tank, but not to the extent that it can't eat. Could it just be a slow developer? I know the most likely explanation is that it just isn't getting to eat as much as the other two, but here is a comparison of the two:
http://www.geocities.com/adoptanapbt/discus8.JPG
http://www.geocities.com/adoptanapbt/discus9.JPG
http://www.geocities.com/adoptanapbt/discus8.JPG
http://www.geocities.com/adoptanapbt/discus9.JPG