Bobo called in to make fun of Sal for not understanding what "within three" meant but Robin quickly noticed that Bobo didn't get it either. So Howard asked what he thought it meant and Bobo answered: "Within 3 people. Not more than 3. It's one, two or three."
After the laughter died down, Howard, Robin and Gary tried to get Bobo to understand what it meant to no avail, reinforcing the fact that Bobo scored an 87 on his IQ test.
Even Sal was able accurately describe what "within three" meant, although Howard thought he had gotten help.