I imagine you can still hear fairly well, possibly as well as a lot of folk who are unaware that they have hearing impairment - however small.
Playing three instruments must mean that you can hear well enough to play well, rather than be blissfully unaware of mistakes you may make!
Playing by ear isn't only about having good hearing, in fact, I guess that those who can do it successfully would still manage with not-so-good hearing.
Your lack of ability in that area is probably down to insufficient knowledge of theory concerning the likes of scales, chords, harmony, and possibly timing. I don't know, obviously, 'cos I'm not watching you try! I imagine you play primarily from the dots, as do so many players, and are a good sight-reader. So often, players are very good at sight-reading but not playing by ear; or vice-versa. If that's you, having improved hearing won't help, but less reliance on the dots will !