I would ditch the upside-down catfish. I have on in my 30g long tank and he is around 2.5", way too big for a 3
gal tank. Also, upside-down catfish are nocturnal, they sleep during the day (and hide), and move about at night.
I would consider returning the catfish and getting something smaller, maybe a few tetras, or a betta by itself.
Walmart sells a 10gallon "deluxe" kit that comes with everything you need except decor, gravel, and fish. I started with that many years ago. Original heater, filter and hood still work perfectly. I think the kit cost $99, everything for mine was about $130.