If the H40 fits you well, you have no inner ear issues and you enjoy the relaxed detail of the H40, you should be happy. Although, if you want an IEM with a better fit (much smaller), no inner ear suction issues, much cleaner presentation, better instrument separation, faster/tighter bass, and better detail, the FH3 is IEM for you. They do have generally similar FR, but the FH3 has smoother yet still detailed treble. The H40 treble is a tad more abrasive.
I’d be happy to write down my observations after doing some critical listening back and forth against a wide variety of music as I did with the FH7 vs FH3 last night.