It is not too late. I have friends who look at me like WHAT your kids dont sleep with you?!! My fav book is Baby wise, then child wise. He teaches you that your marriage comes first. Because that is the one you need to modle for your children who are only with you for a while, while your mate is with your hopefully forever. All of my children slept in our room in a bassinet for 4 months. I then moved them to their crib in their own room. I had to listen and know their cries, but I knew it was time. The first and second night they cried. The 3rd day they whimpered a bit and now my 7 month old..at the time 4 months has been sleeping through the night like all my other children. I also am aware of the 45 min intruder around 5 to 7 months, and I get up and feed. I also do not put my child to bed in their crib while they are asleep or in a simi sleep. By the time of the first yawn, it might be too late. Look for signs and then put them to bed, or nap. I place mine in bed, cover them up and walk out. No, aaawww I know I know validation of some kind. I just do it because that is sleep or nap time and that is it. Baby wise also believes in a schedual and I am not that kind of person, but WOW It really does make baby and mommy life SO easy. WHY is it easy? All of my babies were happy(sleep) easy(sleep) it is all about schedual for them, it helps them feel safe and their bodies relax and they are not uptight and stressed or hard babies. As for your 3.5 year old. YIKES! 3's for me were the worst. Best advice I can give is that YOU are the parent and you say when why how and how much!! You are the leader, not the follower. Be strong, start them at the same time. Tell your 3 year old NOT to get out of bed. First few nights sit by the door and say GO BACK TO BED, then he or she should finally stay in there. BE strong!! All kids think at one time or another their parents are mean.....but it is what is best for them.
Also babies scream in 2's or 3's First cry you pick up, while holding they may calm and then start up again and once more after calming.....it's a chain reaction they can not control.
I have an 8,7,5 and 7 month old they all go to bed EVERY night at 7pm and wake at 7am. It gives me time with my husband to watch a dvd and have a quite dinner. Also my kids are very happy kids. My 8 and 7 are on the honor roll, I think it is because they get the recommended 12 hours of sleep at night.
Be strong and you will do this!! Get your life back!!