That sounds about right, however you might want to kick water changes up to 50% every 2 or 3 days... I kept 1 4" goldfish in a 80L uncycled quarantine and ammonia built up like nuts! I was doing 100% water changes almost daily! Espescially if the goldfish are producing more ammonia than you are removing.
Say your fish produce 1 ppm of ammonia per day.
You change 50%, which brings it to .5 ppm.
The next day its back up to 1.5 ppm.
So you do another 50% water change, bringing it down to .75 ppm.
The next day they produce another 1 ppm, which makes 1.75 ppm.
You change it 50%, bringing it down to .88 ppm.
On day 5 its up to 1.88 ppm.
A 50% water change brings it down to .94 ppm
So on monday you started out with 0 ppm and despite 50% daily water changes it went up to almost 1 ppm by Friday!!!
I think the test kit will help you a lot in bringing your ammonia down to safe levels once you get it.
And to answer your question about the filter... Assuming the 3 foot tank holds 50 US gallons you would need 500GPH or 1893 Litres per hour. Form what I can tell the Aquaflow only does 500 LPH? You can add extra filters to increase your filtration in LPH... the Aquaclear 70 does 1135 LPH which in addition to the Aquaflow should suffice.