I REALLY REALLY REALLY love the marineland 5 gallon hex tank.
It uses the same filter system as the 3 but has a screw-in socket for the light and is a really nice ratio of height and width.
The three gallon one I've had to disable and remove the fluorescent light in because it overheats and melts the plastic casing. Most people haven't had this problem but it does occur.
On these bio-wheel tanks when you set them up to cycle them, don't have the filter card in the tank. Let the water just flow over the bio-wheel for a while. Once the bio-wheel is "wetted" and slows down you can do your fish-in or fishless cycles. If you put the filter card back in too soon you grow bacteria on it instead of the wheel.
There is a small gap that you can drop pellets through between the filter and the front of the top opening, you gotta sort-of toss them towards the front and be off to the side of the mid-spar in there.
As far as quality of construction, clarity, shape, filtration or light power I'm exceptionally happy with the 3... its just kinda small. Betta will get big enough to want five gallons.
The filter card for both the 3 and 5g are sold at walmart "aqua-tech" brand.