On most forums you would think even mentioning UG filters is similar to fish torture. However, I'm old school... or just plain old... and, back in the day, it was the filtration of choice. I've had several tanks using them exclusively and they work great as long as you take and use a gravel suction device and clean the gravel once every month or two.
That said, they require thicker substrate compared to a non UG filter tank. The bacteria survive in the substrate and too little won't give them area to thrive. I keep 3 inches, but anything more than 2 to 2.5 inches would probable be sufficient.
You will need, at minimum, an air-stone in the bottom of the tube to create a draft or pull to cycle the water through the substrate and up the tube. Better would be using a power-head. I have a 55 gal bow-front and I use two 200 gallon power-heads to pull a lot of water through.
All that said, 3 gallons is a pretty small tank and it will take very little to upset the balance of that small of volume. I would look at it as a glorified fish bowl. An air-stone would probably be all you need for the draft tube. Any power-head would be overwhelming in a tank that small. Since an air-stone can't create the draft a power-head does, I would stay toward the thin side of the substrate depth to make sure water cycling through the substrate occurs.
As for how many and what kind of fish, I would compute your surface area and go by that. If it's a circle, take the radius times pi (3.14). You aren't going to have room for much.
