Hi A.,
Congratulations! Here are my thoughts: get some age-appropriate books at the library or bookstore that will help your daughter understand what is going on (pregnancy/new baby in the house). Also, start having a couple of dolls around the house all the time. Use them to familiarize her with the things she will see (changings, naps, feedings, etc), but especially to help her learn how to treat babies, and be accustomed to you giving them attention. Towards the end the dolls should go with you everywhere, use the double stroller, etc. Also, set aside some small gifts for her to open whenever well-wishers arrive with "something for the twins."
One of the challenges that you may anticipate is that you absolutely will not be able to chase a toddler while carrying babies. Train your daughter now to listen to your every word, exactly, so that you can safely go on outings. Figure out how you will make sure that she stays right by you and does not go into the street during walks, etc.
Plan on getting a sitter, or whatever you need to be able to have 1:1 with your daughter after the twins are born. At a minimum, this should be once a week until she is grown. As a preschooler, once a week will still feel like about once a year to her! It is very important that she can count on that "special time" with her Mom, no matter what. It may take the form of a class in something she enjoys (i.e. parent/child swim class), but should occur whether there is class that week or not. It can even be a simple trip to the park. This investment helps to avoid a lifetime of resentment towards her siblings. She will still have times when she is jealous and at her age, she will demonstrate that by acting out. You can still minimize these issues as much as possible. The more you can help her develop her verbal skills during this time, the better. Start with toddler books about feelings so that she has some more words to be able to communicate with you. Help her learn how to express herself verbally as much as possible; this should be a daily focus. That way, when she gets jealous/angry/frustrated (and she will), she has a chance at telling you about it and feeling understood. Teach her how to manage the different feelings in appropriate ways (i.e. taking deep breaths). It will still be hard for her to do these things, so she needs all the help you can give her. Expect that she will show some regressive behavior as she adjusts to the new babies, but you will stroke her for using her "big girl" ways and it will pass.
It is so wonderful that you are looking ahead on how you will be able to help your daughter with this huge adjustment. It may seem a little early, but in this case you are going to be so busy with the physical needs of three babies (I consider a 2 yr old to still need all the atten of a baby), that starting to read now about parenting your small bunch will pay dividends later. I recommend "Siblings without Rivalry," "How to Talk so your children will listen and listen so your children will talk," and "1,2,3 Magic." Those books were very helpful to me. I ended up reading them multiple times, but you are going to be busier so that is why I suggest getting started now. It will also give you a chance to envision some of the things that you know will happen (i.e. there will be a rough day with the twins at some point, and that is exactly when you may expect your daughter to pitch a big one). Anticipating those moments will help you to be ready and know what you want to do.
These children sure are lucky to have a Mom who cares enough to plan ahead! Have fun with your little ones.
J.