(Newcastle Upon Tyne England)
Put 5 goldfish in my new garden pond 3 days ago and haven't seen them again.
Pond and filter been set up for 3weeks.
This isn't a strange or uncommon phenomenon. I have had two such queries in the last week.
Your Goldfish are doing what Goldfish do naturally in strange surroundings...disappear.
Assuming you tested the water to make sure it was the same pH as their water, and you equalized the water temperatures before releasing them, I wouldn't worry too much yet.
If there are no bodies floating on the surface you can safely assume they are hiding.
You mention that the pond is new, so there will be little or no natural food present, so your fish will soon become hungry.
What you need to do is train your fish to realise that you are the source of their food.
Lightly tap on the side of the pond, and put in a small amount of food.
Repeat the next day at the same time and in the same position. The trick here is only feed a very small amount to the fish, not enough to satisfy them.
After a few days or so, depending on how much human contact they have had, one of the braver fish will be waiting for its feed.
After another week, all the fish should be crowding around waiting.
You can now start increasing the amount of food you are feeding them.
Train your fish this way only if there are no predators in the area, otherwise you will be providing an easy meal.
If you don't start seeing your fish within a few days it will be because there is a lot of natural food available such as mosquito wrigglers, or your fish were diseased before you got them and have lost their appetites.