In a 3gal tank without a filter and with live plants-you could maintain water quality with 1-2 50% weekly water changes with light vacuuming in all places you could reach without moving anything or disruption of plant roots...
While the smaller filtered tanks can establish a biological filter or nitrogen cycle-often they are not stable due to limited surface area for the beneficial bacteria and still need at least 1-2 weekly water changes to maintain water quality....
Filters can help to a degree once they establish the biological filter, but until then all they do is remove the larger pieces of debris from the water column-its still in the tank being circulated over and over-out of sight-out of mind....since this is a closed system......
Clean, clear water doesn't always mean good/safe water-its what you can't see or test for that is often what can cause problems.....
Love to see some pics.....