For a 3 gallon tank, i would take a gravel vacuum (aka water siphon-you can buy at petmsart or any other petstore) and vacuum the gravel 2 times a week. You want to remove 50% of the water TWICE a week. SO change let say 50% on Sunday, and the 50% on Thursday. Make sure to treat the tap water with water dechlorinator(removes harmful metals and chemicals from water and makes it safe for fish) BEFORE adding to the tank.
How often do you feed your fish?