It's Always Sunny in Philadelphia Season 18 Episode 3, titled The Gang Gets Tested, sees the group taking a neuropsychological test.
After getting the test results, Dee and Dennis decide to go back to their study days, and they're pretty much surprised to find Mac also there. On the other hand, Charlie decides to get more tests done after his results come back inconclusive. Out of everyone, Frank seems to have the most fun as he takes a college student under his wing and helps him explore new experiences, which the student is clearly afraid of.
By the end of the episode, the gang realizes that the test doesn't really define them, and they simply brush off the results and move on.
It's Always Sunny in Philadelphia Season 18 Episode 3 recap: The gang heads back to college after taking the test

It's Always Sunny in Philadelphia Season 18 Episode 3 starts with the group sharing the results of their neuropsychological test. And Dennis and Dee get positive results for ADHD. However, Frank doesn't care much about these tests and tells everyone to believe in who they are and not to focus much on the results.
The episode shows Dee and Dennis not accepting their results at first, but there are multiple moments throughout the episode where viewers see Dee getting distracted and not able to come up with proper comebacks, something that she was able to do effortlessly in the premiere episodes of Season 18.
As a result, Dee and Dennis decide to go to Penn to study, and to their surprise, Mac is already there. Additionally, Frank is also there, who wants to recreate the events of Back to School in real life.
It's Always Sunny in Philadelphia Season 18 Episode 3 recap: Frank's Back to School plan goes too far

In It's Always Sunny in Philadelphia Season 18 Episode 3, Frank takes a student under his wing who turns out to be shy. Frank decides to give the student some new experiences, which include breaking into the dean's office without realizing there are cameras, taking him to a party and helping him find a girl, and eventually pushing him to take a dangerous dive.
In the end, the student jumps into the pool, and Frank checks on him only to find that he is unconscious. As for Mac, Dee, and Dennis, the siblings see Mac studying with another student and believe that he got a study partner. But it turns out that even Mac is struggling, and on top of that, Dennis puts a Tootsie roll in his ear, which makes the situation more awkward for Mac.
It's Always Sunny in Philadelphia Season 18 Episode 3 recap: What does Charlie do with his test results?
Charlie goes for another test, and after listening to his answers, the lady asking the questions calls for other professors. After the other professors arrive to question Charlie in It's Always Sunny in Philadelphia Season 18 Episode 3, they are left impressed. At one point, he even says that the woman sitting in front of him has two pencils in her left pocket. He figures this out because of the way she walks, as she steps lightly on that foot to avoid getting poked.
However, Charlie has no interest in knowing what condition he has, as just when the professors were about to reveal what condition he has, he just walks away. On the other hand, Dennis, Dee, and Mac don't find their names on the result list. They soon realize that they were never enrolled in the college in the first place.
The group reunites with Frank and Charlie, who tell them that he doesn't need anyone to tell him who he is. The episode ends with the gang believing that the test was pointless and didn't really define them.
It's Always Sunny in Philadelphia is streaming on Hulu.