My son doesn't have a stepmother yet, but may someday. In thinking about what I'd like his stepmom to know, lots of things come to mind, and I'm sure those things are somewhat age-specific, as in, the things that are most important when he is 5, may change when he is 15.
It's a given to me that stepmom would be privy to any and all health related needs (i.e. my son's peanut allergy and mild eczema) so that she can take the necessary precautions, and administer the necessary care. That isn't something that I *wish she knew.* That is something that she would absolutely be made aware of the instant she became a caregiver to my son. So while those things are important, I don't think they fit in the category of what I wish she knew... I mention this only because I see that this hasn't happened in some cases, which I can't even fathom...
As for the things I'd want my son's stepmom to know that might facilitate a better relationship, well, I think that first and foremost, I would want it to be cleary understood by all parties, smom included (sorry, lol, I couldn't resist the smom thing.. ;) that my son did not CREATE or OTHERWISE ASK to be in this situation, living between 2 families/households, and with that, the understanding that it isn't always easy for him. And while this is probably something the stepmom 'knows,' I'd want to be sure it was *understood.*
Secondly, I think I would want stepmom to understand that while I would expect my son to be respectful toward her, and to abide by household rules (and I'd be sure to share our 'rules' so that she would be aware of what my son is used to on a daily basis), I would expect her to defer any disciplining to my son's father. I believe that this is best for EVERYONE involved. Stepmom doesn't have the bond with my son that his father does that allows him to discipline him without damaging their relationship. That is not the case with step parents. My fiance does not discpline my son. He will give gentle directives, and redirection if necessary, but no real discipline. That's deferrred to me. I would expect that to be the same at his father's home.
Lastly, since you asked for 3 things, I would want stepmom to know that I desire for all of us adults to be civil, cordial even, so as to facilitate the most loving, easy, peaceful environment possible for my son, and for everyone involved. There is absolutely NO animosity whatsoever between my son's father and myself, so if we were the ones with the relationship issues, yet we are working harmoniously together to parent our son, then there is no reason that the rest of us shouldn't get along as well. My son deserves to be at peace, and ideally, to feel *loved* by all the parents and step parents in his life. But I know I can't force someone to love him. But I do hope and pray that any 'steps' that come into his life have the heart to love him.
So those are my 3 wishes.