I am a SAH mom of 4, daughter 16, daughter 2 1/2 and baby girls 4 mos. The first 2 1/2 mos were the hardest. Sleep? What's that? My husband and I got advice from many parents, most helpful was from mom & dad of quads- during the night feed them both at the same time. By doing this we gained 2 hrs sleep the first night we tried it. Also, check with doctor regarding rice cereal our girls were much happier once their bellies were full regularly. This help put us on a schedule.
Working out a schedule and keeping with it has worked for us. Of course as our little girls grow we have to change our schedules here and there but it seems to be working. Especially when it comes to naps during the day. And once we moved them so they weren't co sleeping they take longer naps and sleep all night.
Ask for help. I know it's hard but who doesn't like to spend an hour or two cuddling a baby (or two) that's just enough time for a nap and quality time with another child. My family & friends were very helpful even w/housework.
Another thing that has helped me is getting out of the house and going to a friends. She also is a sahm with a 3 yr old and a 3 mo old. We compare & swap stories.
Make sure you have time for you. Find someone to watch the kids for one day and get out and do something for yourself what ever it is that you enjoy.
It does get better hang in there.