Moving turtles outdoors and hibernation

I am now getting ready to move my turtles to a specifically designed outdoor pond. Before I do I have some questions. I have three turtles, currently about 6″ long. Is there a good depth for turtles, looking around three feet seems to be mentioned alot? Is that deep enough for them to be happy? Can anyone suggest good pond building tips for turtles that I should include in the setup I am making for them? Size is not an issue as I have copious garden space. I was looking at biulding something about 24 feet wide by 18 feet long and about three feet deep. I assume gentle sloping sides is a must, and that mud for digging around the edges is good, plus a stoned area for basking as the stone would hold heat. I thought I might have two areas divided by an island and fence the edge to keep unwanted guests out. I thought some goldfish, would give the turtles some exercise. Any ideas? How do I hibernate the little fella’s? Can they be left out during the winter? I live in the UK and my minimum temp is about -4C. I will be using a floating heater to keep surface open to avoid gas build up. If they cannot stay outside, how do I hibernate them? Thanks for all advice. Scott