Many poskim say to tell on the 3rd date. Others say there no “vort davka” in the 3rd date. Its an inyan that you can’t get the girl to fall for you and then tell her you have a mood disorder. 3 dates 6 hours each is different than 3 dates 2 hours each. I think we’d all agree on that. And a 22 year old bochur’s 3rd date is different than a 40 year old baal habos’s 3rd date.
That being said, of course I’d tell the girl about my mood disorder before it gets serious.
I wish there was a book of stories of people on medicine who got married.