Undergravel filters aren't that good in my opinion. =/ It'd get a Tetra Whisper Filter (they make them for 3 gallons) or something else if you can. =]
Why don't you go for the
Hawkeye 5 gallon hex? It cost about 5 dollars less and they'll ship it to your house for only a dollar. It's bigger (you could put more shrimp in and a snail) and it comes with a MUCH better filter! =] I have the same filter that comes with that system and it works wonderfully! =]
Also there are many experienced users on here with the Hawkeye 5 gallon hex and they've all given wonderful reviews! =]