I don't believe in "sticking to your guns" at night and "staying firm" because all that does is set up a power struggle with the child unable to sleep (up for hours - you can read that on mamasource) and parents can't sleep either. I know that's what all the books and magazines say, but that is coming typically from pediatrician's and mainstream writers (not anthropologists or child development experts, people who know and respect how human beings are wired. We are afraid of the dark. We need human contact.) There are many POV's but only one is usually pushed forward... the "make them independent early" POV is what is encouraged.
Here is what I have always done. At 2 I moved our son from our bed to his twin bed in his room. At age 1, I moved our daughter to her twin bed on the floor right next to her brother's bed. I always stayed with them until they conked out, then left the room.
That accomplishes a few things.
1) They know their bed is THEIR bed.
2) They associate the good, yummy, snuggly feelings with their safe place (their separate bedroom).
3) You don't ever have to carry them from your bed to their room (why?)'
Mary Sheedy Kurcinka explains in her book SLEEPLESS IN AMERICA
http://www.parentchildhelp.com/
that children (and adults) need to be in the GREEN ZONE (relaxed) to feel safe and go to sleep. If they are in the RED ZONE (in a power struggle with parents, anxious, fearful) then guess what? "Fight or flight" kicks in and the child is literally unable to sleep.
I know that if I lie next to her after books, and she feels safe, she is out like a light. When my son was younger, and I was ITCHING to get up out of room, he could feel my tension, frustration and anger and it would keep him up. He was afraid I would leave. Once I relaxed (basically surrendered to the moment) he was out in 5 minutes.
if the little one wakes up and your 3 yr old starts to cry, rub her on the back and say, "I'm here, you are safe, go back to sleep, I am going to help your sister." Talk to her about the nighttime during the day...
My daughter is 4 now and after books, I lie with her in her twin. I go back to my room. She sometimes wakes up at 5 or 6am and quietly comes looking for me. I take her hand (no arguing, no tension) and walk her back to her room and quietly lie down with her. That helps her go back to sleep. I usually just stay at that point. Sometimes she cries in the middle of the night and my 8 year old usually doesn't wake up.