You should explain to each child according to their age. With older children, you might have to answer many questions. In your case, as your child is three years old maybe, you could tell that mommy has a baby in her tummy. And the baby would come out in a few months. Younger children naturally develop a bond with your tummy(baby). They tend to touch and feel the movements and they really get excited to talk to the baby in the tummy. They get involved in seeing the ultrasound reports and soon they can’t wait for the baby to come out. Just make sure that you include your son in conversations about the baby. He should not feel in-secure and left out. Older children mostly are curious and they ask a lot of questions; you and your partner have to first decide how much scientifically would you want to answer them. That totally is your choice; whether you tell that God put the baby in mommy’s tummy or dad/ mom decided to have a baby!
We had our second one when our elder son was eight. It was very easy for us to tell him, in fact, he was the one who wanted a brother/ sister in the first place. Having second child is a life-toppling experience. You realize a lot in the process and as parents we actually understand the meaning of real parenting. We learn to deal every situation without anxiety and naturally we become more composed. As a family it is an exciting experience. Preparing for the new arrival of a baby with your first child is fun. Make the most out of the celebration!