Originally Posted by fish monger
... Three small goldfish in a reasonably maintained 5 gallon would stay healthy until the switch to a pond. ...
I actually have to disagree on this. Simply because of size. Building a pond takes a while and is usually done in warmer weather. Goldfish grow fast, and in a month or two they will outgrow the room in a 5 gal. With sufficient water changes the water could be kept clean enough, but due to the growth rate, they will soon simply get too large to be comfortable housed in such a small area for that time.
Getting the goldfish to the 20 gal as soon as possible is their best chance. However, if you can find a huge plastic tub that is larger, that would be the best solution.