I found that when the 51/2 and 3 year old girls who share a bedroom with bunk beds stopped taking their afternoon naps that bedtime was a lot easier. It took a lot of practice to get us here, but they usually go to bed by 6:30pm and wake up at 7am. They occasionally wake each other up to play in the morning which I am OK with as long as they don't wake me up. The constant rountine has really helped - reinforcing it every night with basically the same rountine - dinner by 5ish, bath 5:30 bed by 6:30. If one of the kids wakes up early (before the sun rises) I explain while they are still in bed that they should lay in bed quietly until the rest of the family wakes up - that it is still "night night" and we don't wake up until the sun is up. Again, practice.
Oddly, I have discovered that the earlier they go to bed the later they sleep in - go figure! I think a lot of this has to do with getting them used to sleeping and with them getting older. I've also got a 1 year old who only takes an afternoon nap and is in bed by 6pm he wakes up earliest - at 6am so I am hopeful that as he ages he will sleep longer. Needless to say my bedtime is around 7:30/8pm.
I noticed that sleep has been one of the most important things to get down. My kids are happier and not so grouchy in the afternoon. I think this comes from them knowing that they can play hard but soon they will get their dinner, bath and a good night's sleep.
Good luck with #3 - it's the magic number! I found it a bit challenging at first with the family transitioning but again it is all about the routine as they get older. And, - the good news, with a solid routine it does get easier.