If you're in 7a, aeration is good, it mostly will keep a hole in any ice that might form. For three small fish in any small pond if you have a couple feet of depth you probably don't need a heater, the fish go dormant so don't need a lot of care through the winter. ice forming over the top can block oxygen exchange and would be the only issue other than water levels. gold fish are pretty hardy, and odds are that you have a male and female, so get ready for lots more fish in the not too distant future.
It starts with a small garden pond and just a couple fish, and can easily grow to larger and larger ponds with fancy fish, waterfalls, plants, and your own personal paradise, have fun.