It is probably easiest to let your children know they were adopted as early as possible, from their first experiences with observing pregnant women or hearing references to sexuality, pregnancy, and childbirth. A parent can lay the groundwork for the idea of adoption by pointing out that a pregnant woman has a baby inside, and that sometimes the woman raises that baby herself and sometimes another women raises the baby instead. Since small children think concretely and because the facts of life are quite complex, it takes many years and many iterations of explanations for youngsters to get the facts right. So universally, children display ideas which don't match up with the facts very well--such as the notion that women become pregnant through something that they ate, or that babies are born through the belly-button.
So little tots are likely to have fairly inaccurate ideas about adoption, even if parents have "explained" it. This is fine. The main thing the parent wants go get across is that the parent longed and longed to have a baby, and that the child is that longed-for baby that fulfills the parent's deepest wish.
The best way to deal with the complexities of adoption is to tell the truth--or, a version of the truth which is simple enough that the child can absorb it. It is not clear to me whether your three daughters are siblings to one another and whether they were all adopted on a single occasion. Whatever you may know about their circumstances prior to adoption, I am sure you can weave some of these circumstances into a story. You will find yourself returning to this story over and over, since your daughters as they grow will be developmentally ready for different aspects of it at different times.
The story of the child's origins--any child--form a theme of identity which is conveyed a thousand ways throughout childhood and in fact throughout life. Every child has a thousand questions about this origin--whether adopted or not. For this reason, the facts about reproduction and the facts about adoption cannot fit into a single speech. The dialog between parent and child is ongoing and has a life of its own.
No one has all the answers to those "baby" questions therefore. Why am I here? Why am I myself and not someone else? These are questions that no one can answer. Your role is to love your daughters, and to share with them reasonable information about babies and the various ways that babies may enter new families. Like all mothers, you can explain that you felt love at first sight, because you knew that these children were meant to be yours.