Congratulations!
Keep her involved from the start.
My toddler was about 18 months when we found out we were expecting twins. He came to the scans, we talked about babies a lot with him, showed him the newish babies of my friends. He helped choose things for the babies and we talked about what he thought their names would be.
We made sure that he wouldn't feel that they had taken anything from him so he kept his own cot etc. when we were decorating a room for them we also did some decorating in his.
When the time came he was really excited. DH bought him a 'best big brother' t-shirt which he wore when he came to the hospital to visit them. He didn't want to see me, only his sisters 
There have been times when he has been a bit clingy, especially in the first few months, but he absolutely adores his sisters. I have always made sure he had some time with me each day when the babies were napping. He helps me choose their clothes and helps wash their bottles and make up their milks.
He goes to my In laws one day a week, and nursery 2 mornings so we kept his routine the same as possible. We asked them to only speak to him about the babies if he mentioned them, that way he could have a good break from the twins.
Also, let her see you holding other people's babies so she gets used to it.
It'll get hard looking after the toddler as you get towards the end of you pregnancy, but having two babies and a toddler has been so much easier than being heavily pregnant with a toddler!
Good luck 