I was curious to read the responses that you received for your question because my almost 3 year old daughter is doing this and we are about a week or 2 from expecting our 3rd child! My daughter was really too young to understand when we had our 2nd (he's 15mos now)... she was still in a crib and slept with no issues. Now, she is in a twin bed and life has changed!! I was wondering if she was reacting to this new baby with her sleep issues too. We talk to her about the new baby, and she seems excited to share her toys with her, so it seems like her feelings for this new one are positive... but that's all we have done to "prepare" her this time.
So, we have gotten rid of the midday nap... (except on days that she seems totally exhausted)... and replaced it with a "rest" time where she can read books or watch a movie or something quiet. This is when I try to catch up on some housework too.....
This has made going to bed a bit easier because she is tired and ready for bed, but she still is waking up at 3 and doesn't fall back asleep until 4ish! We have to lay down with her and it is so tiring! She plays the, "No! I want Mommy/Daddy game" then too.. we never know who she is going to want....
So, recently we were thinking that possibly she is hungry at night, so we are trying to feed her an after-dinner snack to try to fill up her tummy before going to bed. (She is also a picky-eater at dinner-time.)
Some nights this seems to help, so I thought I'd add that suggestion for you to consider... We also make sure she has a sippy cup of water right near her pillow in case she gets thirsty.... stuff like that.
Good luck & congratulations on your growing family!