Why only three? I don't know... maybe there's some kind of time collapse field in effect when you get to the past (the past can't handle the paradox of holding a future you and a present you in the same time thread), that makes it impossible to hold onto more than three pieces of information in the detail required to alter events. Just pick three.
I think I know what my three would be, but I also know that information, once out on the web, doesn't go away. Despite the knowledge that literally no-one reads this stuff, I can't help but think that someday someone I know will read it, and the fallout could be life-altering... almost as life-altering as going back in time and telling myself three things... oh what the heck.
1) Sex isn't that big a deal. I used to make a big deal of it, and I was told that it was a big deal. Obviously, if you're a teenager, or if you know teenagers, you want to stop them from having sex with each other as much as possible, and in any way that you can, but here I am in my mid-to-late thirties, and I'm saying to all you responsible adults out there: have sex. Have lots of it. Enjoy it. If you make babies, take care of them. Enjoy them. But have sex.
2) You have Kleinfelter's Syndrome, which means a lot (read a book about it), but the upshot is, you can't get anyone pregnant. See #1. Go to it.
3) Date lots, often, and for a long time. You don't need to be married, unless you want to raise children. Since this is impossible for you, unless you want to adopt or get a donor, take all the time you need to date, date, date, making sure the way she expresses love is the same way you do, or at least close. This opposite thing sucks.