I don't really understand your question.
Do you have a waterfall in you backyard that is spring fed and you want to know if you can put koi in there and if they will be a problem for the turtles and frogs there? If so, then don't do it.
Do you want to build an outside cage and make a pond with waterfall in it that will be cycled by a pump? If so, the koi will almost certainly try and eat your frogs and the turts will nip the koi and frogs too given half a chance.
Or are you going to try and stock a natural spring fed pond with turtles and frogs and koi? If so, you should really not do it, as you will be allowing the turts and frogs to escape into an area they are not necessarily native too and risking disease spread.
You could put EWD's in an aviary with koi in the pond, and turtles too if the pond is large enough to allow everyone to hide from each other.