You'd probably need something about 100 gallons for three goldfish. Deeper is better for ponds (18" minimum), as deeper ponds keep more stable temperatures. If the summers get really hot, try to pick somewhere that's shaded from the afternoon sun. According to the USDA map, your winters can get down to 10 degrees F, so you may need to bring the fish in for the winter.
If you don't have big freezes in the winter, you could try livebearers instead of goldfish.