Are these natural in English?(←Can I ask like this?)
a)If you are happy, then I am too.
b)If you are happy, then that is already good enough for me.
c)I'm happy if you are feeling fulfilled
"Are these natural in English?" You can ask it like that. You could also just say "Are these natural?" or "Do these sound natural?"
As for if the sentences are natural, all of them sound natural to me. Good job!