33 gallons is certainly too small for three goldfish, no matter how well filtered. Sufficient filtration will eliminate ammonia and nitrite, but will not eliminate nitrAtes, which can be harmful in high levels. Nitrates can only be eliminated by healthy plants, water changes, or a nitrate filter (the latter is hard to set up and run in a freshwater system). The only filter system that would eliminate nitrates is a drip system, where the water is being constantly changed out for fresh de-chlorinated water. If you would like to set something up like this I can help you, but it wouldn't eliminate the SPACE issues for your goldfish which is probably a concern.
Are you sure you can't upgrade to a larger tank? Used tanks can come cheap if you keep looking places like craigslist.
There is nothing wrong with them being a pond as long as the person knows how to overwinter and care for goldfish.
As for making a pond, you don't HAVE to dig. I currently have a 300 gallon trough in my backyard that is a pond. It doesn't take up as much room as you'd think. You can't overwinter in something like that unless you put a pond heater, but you could always bring the fish inside temporarily over the winter. Heck if you have enough room you can bring the whole trough inside, but you would need to cover it or else your house would have a ton of humidity.