Co-sleeping to Crib at 9 Months

It really depends which approach you want to take, as there are some very good guides out there, but they all take a slightly different philosophical approach. My advice would be to get a feel for the various approaches and then just choose one, based on what you know about yourself and your son. Intuitively, you will know which approach will likely work best for you and him. Sleep training is going to be tough at times, no matter what, so it really behooves you to be as resolved as possible before you begin. Giving up and going back and forth will only be confusing to your son-- though a little bit of that is inevitable-- we are all human and it's hard to see our babies struggling to adjust to a new routine, so the temptation to give in an go back to the old method is strong-- even though you know it is no longer working for either of you.
My sister swears by The No Cry Sleep Solution. Another friend found Ferber best. (I thought his ideas were terrible before I actually *read* the book. It gets a bad rap but it is actually full of flexible and compassionate options.) Same goes for Weissbluth. I haven't read The Baby Whisperer but another friend says she found the routines in it very helpful. I read about six different books and finally had to chuck five of them out the window and stick with one, because they do really contradict each other! Best of luck!!

Read Dr. Sears Baby Sleep Book, The baby Whisperer and The No-Cry Sleep Solution. These were my life-savers when trying to learn about infant sleep and then use that info to teach my 7 month old to sleep. Life-savers.
My oldest co-slept with us till he was around 6 months old and then we all began to wake each other and I felt the same way-where the heck do I start and how do i do this!

I had no clue how infants slept or routines. My mom passed away and we were living away from family so it was on me to figure it out.

All of these will give you invaluable tips, tricks and info about teaching baby how to sleep. And yes, they have to be taught. One of the things I did was get baby to sleep while nursing or rocking, made sure they were into good sleep (like 10 minutes after they passed out), would keep baby close and lay baby down and if baby stirred would keep my hand on baby and put my head down on the side of the crib till they passed back out. They I gradually didn't have to do that anymore. It also helps (especially this time of year) to warm the bed with a heating pad or I made a rice bag that I could heat in the microwave. Just make sure you remove it before putting baby in. Not hitting cold sheets makes for an easier transition too.

Try a sleep-sack for him.
Amazon has these.
And let him have a lovey....
And make the crib comfy. For my son that is what I did. He also had a Fisher Price "Ocean Wonders" crib toy, I put extra slight padding under the fitted sheet of his crib (with a towel) to make it more comfy, I put some baby safe stuffed toys in the crib with him (which he liked) etc. My son LOVED his crib.... that way.

Bear in mind, 9 months is also a "growth-spurt" period in a baby... so feeding amounts and frequency WILL increase....
and sometimes, baby is also teething at this age too...
so many things going on ....

Next: for your 3 year old... you simply explain to her... what a baby is. When I had my 2nd child, my daughter was about that age. I simply explained to her... what a baby is, that it cries/feeds/I breastfeed/that babies wake all the time/they cry/Mommy feeds him etc. I explained to her... she does NOT have to 'worry' because it is MOMMY's job... to care for baby... and it is 'normal' that babies cry and wake.
She understood just fine, and my explaining to her about her baby brother.... helped GREATLY. At each age juncture, I also explained to my eldest, about what her baby brother can or cannot do, yet. Per his age. This helped her and she then understood and then got a good idea about why her baby brother is not like her and a "baby." At each age of development... I explained... to my daughter, about how her baby brother is changing/growing/developing etc. So that SHE can understand and it really helped.

You can try by having him adjust slowly. Does his crib turn into a toddler day bed? You can push the crib completely up to your side of the bed (minus the rails) that way he is still next to you, but in his own space. Once he gets used to this, you can put the side back up.
Or just push the crib next to you by the bed so he can still see you. I do recommend a sleep sack, as well as a stuffed animal or blankey he can cuddle. Do it now or he will get too big and need to co-sleep at 2.5 years old like my son!

Get him outta there quick!! I remember my son beating me up in his sleep. We actually had a very similar situation. My son was sick for 3 months so I put him in bed because I just couldn't get up over and over to nurse. I'd wake up and not be able to go back to sleep and he seemed to wake and cry much less when he was in my bed and not his own. I went cold turkey. It's not so bad for him but it will cost you a few stressful nights! I put him in his crib and would come to him when he cried. We'd rock but I wouldn't allow him to nurse. If you are still including a feeding in the middle of the night just stop letting him nurse except for that time. It's tough for about a week or so. I'm not a 'cry it out' mom because it makes me batty to listen to him scream! By stopping the nursing he'll begin to self soothe. It's not over night and you CAN'T let him back in your bed. It will backfire! I was surprised how quickly he did get used to not feeding. He still needed comforting as many times a night but I was able to start going in his room and soothe him without getting him out of his crib. It was a slow process but it worked out to be the most humane way for both of us. I must add that I was one of those 'pre moms' who said I'd NEVER let my kid sleep in my bed. That attitude didn't stick long when I actually had a kid who kept me up all night!! I've found parenting quite humbling! I've also found that the method you use to keep mom and baby happy and healthy is different for everyone and every baby. It's been very hard but I've put aside my know it all ness and adopted a new 'what works for us is ok' policy! It took some time but it was worth it to get my bed back without feeling any guilt over the separation. Good luck and even when you think you're going to pull every hair in your head out hang in just a little longer.