UMMMMM..I have not done this, but I would think that you do it the same way that you get ready for a biological baby.
You let her help you to prepare the nursery, do the laundry, the shopping--let her participate in the shower-- and prepare!! Let her pick some things out for the baby. It sounds like you are not giving a three year old enough credit. And pick out and hide a present for her from the baby when the baby arrives. Make a big deal about her being the big sister. The more time that you give her to get ready the less sibling rivalry and adjustment issues you will encounter. You would not just dissapear and go to the hospital and come home with a baby without telling your child what was happening would you?
You have things that you do when you are having a child that can mark a new addition and it is the same no matter how that addition comes. I would think that the only thing that would be hard for her is if the adoption fails in some way. I would be cautious about that. I have four children and all of them were terrific about preparing for a sibling.
My now seven year old, when told that a new baby was coming told his older siblings, "Great, let's go outside and wait for the new baby to come" and the eleven year old explained that it would take longer than that.