I bought these...
https://shop.hoosierpondpros.com/Vertex-9-Single-Air-Diffuser-VertexDiff9.htmThey have 3/4” Male NPT threads on the bottom. So, yes, they just screw into the threaded coupling you see in my photo.
I did not add a check valve. They may already have one built in.
I can not say how a three diffuser set up would work. It would certainly be easier and cheaper to put together, but you would have less versatility as you could only put it in one spot. I like to have mine in three different areas for no other reason than aesthetics.
One advantage to having multiple lines going into the pond is if you want to run one of the diffusers during the winter time in a shallow area. This is not too important in our milder winters as it is up north.
If you have fish big enough to pull you under...we need to do some fishing!
