Your little girl is beautiful and is 100% a female. If your birds are mixed genders, please be sure that you never do anything to promote breeding, especially as they are siblings. Additionally, with three budgies, it is possible that one will feel left out in the flock as they mature. Keep this in mind in case you need to house them separately later on! Here are some links with important information:
Additionally, please read through the forums' wealth of other stickies and articles to ensure you're up to date on the best of budgie care. If you have any questions afterwards, please do ask as we'd love to help.