I am so deeply sorry for your loss.
The person with whom this child will now be living, will want to be the person who tells her. At three, children understand that when people die, they go away forever. So you can use the phrase, that "they died".
Depending on everyone's beliefs and how you want to raise her - you can tell her that Mommy and Daddy died because.... either God needed them with him, or whatever you feel necessary.
She is going to go back and forth for awhile wanting to know when they are coming home - even though she understands. She may regress for quite some time - and go back to bedwetting, thumbsucking, wanting to be fed - this is normal - children go back to a time when they felt safe, emotionally in order to deal with trauma. Placate that... she will come back to her regular behavior.
DO: Get books written for preschoolers about parents dying. Read them before you purchase them, as they all have a different take on where parents go -- but above all, be honest. Explain that it is not fair, that everyone dies someday, but they died too soon. Do tell her it was nothing she did (kids blame themselves, even if they don't tell you that), do tell her they love her and wish they could be with her. Do keep as much of thei "stuff" as possible for her for when she grows up. Keep all pictures, some clothes for her to use as comfort, and some bedding for her to sleep with. Allowher to grieve.
DON'T: talk about how her parents can see her, don't talk about ghosts, don't talk about death as scary, don't talk about hell, or devils - even if you believe in those things, she is too young to deal with that and it will turn into a fear or phobia.
If she begins to have daily nightmares and can't manage her emotions, take her to a grief counselor for therapy - it is common that children need this.
How else can I help?