[Geomancer]

Let the poor guy do his business in piece I haven't heard of that being a problem, and that's what it looks like for a lot of larger fish.

However, I will say that you can't have a catfish with the betta in a 3 gallon tank...

If it's a cory catfish, they are schooling and need to be in groups of at a bare minimum 4, but they do better in larger schools. Even if they were not 3 gallons is tiny and isn't really adequate space for even a betta alone. I know a lot of people around here say it's fine, but I'd personally say 5g is the smallest for a single betta.

 

[Geomancer]

The problem with the cory is not a compatibility one, it's a mental health one.

Cory cats are a schooling fish, they need more of their own species to feel safe and secure. As the only one he is likely stressed, unhappy, and can develop aggression issues. In nature there is a reason they are found in large groups, it isn't by accident. "Safety in numbers" as the saying goes.

Stressed fish are more likely to get disease as stress weakens the immune system. In addition they will release hormones that can affect other fish as well "the scent of fear" as an analogy. Stress also will shorten their lifespan, just as it will in us humans.

You can view the profiles here depending on the species you have, the link is on the blue bar at the top of the page, second one from the left. Just expand the Catfish list to see them, for example Pepper Cory. Or you can hit the order by scientific name button at the top of the page and look for Corydoras. Those profiles will give you a lot of information on how to keep your fish as happy as possible. Happy fish = healthy fish.