Three pieces of advice for you...
Answering questions about the pregnancy. In general, it's best to be factual, concrete, and simple. Young children ask what they need to know at the moment, and what they are ready to understand. For example, if your daughter asks, "How did the baby get into your stomach?" you could answer, "He is not in my stomach. He is growing in a special place called a uterus." You don't need to go into any more detail, unless she asks for it.
Expect ambivalence. Don't be surprised if your daughter has a lot of mixed feelings. She might be excited, eager for a playmate, worried about being pushed aside, or she might not show any strong feelings at all. Often the new baby isn't "real" for a young child until after the baby has been in the home for days or weeks. The realization that the new baby is staying often comes as a shock.
Invite involvement, but don't push. A three-year-old might be very interested in learning about feeding, diapering, and bathing. Having a baby doll can be useful for this purpose; this goes for boys as well as girls. Children who take on a helping role often develop closer relationships with their younger siblings, On the other hand, everything should not all of a sudden revolve around the new baby. By continuing to do some of the things that your older child likes to do, you let her know that she is still important.
Other relationships. The biggest challenge for older siblings is to give up her exclusive relationship with you. It helps if she has strong relationships with other adults, such as her other parent, or her grandparents. More time with them is a bit of a consolation for the loss of some of your time and attention.
Keeping some things the same. The birth of a sibling brings with it profound changes in a young child's life. In the midst of this, it is comforting to have the same meals, playtimes, naps, bedtime routine, and bed. Predictability helps a young child feel more in control. If there are special things that you do together - such as reading books, or going to a particular park, let your daughter know that these things will not change.
D. ;-)