Don't correct her. Tell her she has a Mama and Dada in heaven, show her pictures, let her grow up knowing them, and tell her that you feel so blessed that you get to be her Dad too and how much you love her. If she knows that you love her and would do anything for her like a biological parent, she won't hate you for it.
I wouldn't wait until she is older to talk about her parents though. At three years old if you never talk about her parents it would just confuse her. Talk about them now, put a picture in a plastic picture frame and teach her to kiss it goodnight or simple things like that, she won't fully understand but when she starts to have questions she won't be too overwhelmed or shocked to be able to ask you those questions.
You can refer to her birth father as her birth dad, or Papa, if you don't want to confuse her with having two "Dada"s.
I'll be blunt- I'm sorry to hear that your close friends lost their lives and a little girl has to grow up without her parents, sounds like a very tragic situation all around! That being said, they chose you to be her parent if anything were to happen to them and they weren't able to raise her... that IS what a god-parent is, so no you aren't dishonoring her by raising her, or for her calling you "dada. Here's the thing, you are her dad... you are the one who is raising her and the only father she will ever know so I don't see why it's wrong that she looks at you that way, and you better get used to it because she will be calling you that a lot in the future!
If it makes you feel any better then you can always tell her about her parents when shes a bit older, you can show her pictures, you can play videos of them for her.... you can keep the memory of them alive for her, this way she knows her parents werent taken by choice and that they loved her a lot. Be blessed.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.