There are two good reasons to wait 3 days (or more) between introducing new foods. One, it takes a previous exposure to have an allergic reaction. So if you feed banana today and then banana tomorrow she could have a reaction the 2nd day. So in the three day time frame baby can have anything known that she tolerates and at least two tries of the new food.
And two, that way if baby does have a reaction you know what she is reacting to. I never dreamed my daughter would be allergic and she reacted to cranberries (rash), milk (rash), and eggs (hives). The cranberry and milk was red around her mouth and where it touched her (like if a bit got on her cheek). Eggs made hives all over her little face. Very scary and not something you want to mess with. I say better safe than sorry and if you have any history of allergies I would keep benadryl in the house just in case. Not trying to scare you, I just wish someone would have told me more. I did lots of research before hand but like I said, I never thought in a million years that she would have food allergies.