If you’re looking for a starting point, there are a couple very simple games in the jme3-examples project: CubeField, RollingTheMonkey, and WorldOfInception:

https://github.com/jMonkeyEngine/jmonkeyengine/tree/master/jme3-examples/src/main/java/jme3test/games

RollingTheMonkey is the best of these, in my opinion: worth playing for a minute or two.

My one open-source game is (barely) playable, but lays some groundwork for a more sophisticated project:

https://github.com/stephengold/jme3-maze

If you’re looking for a client-server game, there’s MonkeyZone:

https://github.com/SkidRunner/MonkeyZone