First, welcome to A. A. , now on to the questions.
Generally its recommended not to mix native or wild caught specimens with tank raised specimens. The wild caught have usually built resistance to a number of different diseases found in nature, but may carry that disease into your aquarium. I would recommend a seperate tank for the wild caught natives.
While all three specimens are native to N. America, it doesn't naturally follow that all three need the same water chemistry requirements.
I would also be concerned with the metamorphosis of your Spring salamander and accomadating that change. Once it gets used to feeding on provided food, it may not do so well when you release it, and since they are only semi aquatic, you will have to release it or provide another suitable environment.
The clams are filter feeders, and may or may not get enough food in your aquarium, they will not survive on fish food.
Also be aware of local laws; are there any local laws that would prevent you from collecting from the stream?
Just some food for thought.