Please ID for me.
They are only about 3" long but are defending what appears to be a nest.
Because of their shape and dark colored opercular flap (ear flap), they certainly seem to be some kind of sunfish -- stunted bluegill, green sunfish, some kind of hybrid, etc.
The best way to identify them is to catch one using to use a trout hook, like a #10 fly hook, with a small piece of redworm. Put a small splitshot sinker about 12 inches above the hook and toss it into the nest.
Blurry image, but the fish I see mid-frame, moving right to left appears to be a bluegill.
Aren't they a little small to be spawning/protecting a nest?
Not if they are stunted bluegill. That is why catching one for positive identification is important.