E.M.
J.,
My 3 year old son wakes in the night as well and we just keep sending him back. At this point I don't always have to actually get up I can just tell him to go back and he usually races back to his room, but it took us a while to get to this point.. My only advice is what you already know, just keep sending them back to bed. Unfortunately, the next few months will most likely be rough for you, but try to start now so she doesn't connect sending her back to bed with a new baby. I don't suggest waiting until the baby is on the scene and you will have little patience.
Good luck!