Before following Laurie F's advise on using Babywise, please read the following reviews: http://www.amazon.com/Becoming-Baby-Wise-Reference-Worldw...
Babywise has been discredited by parenting experts, medical experts, and even religious experts. It does "work" for some babies, and these are held up as examples of how great the system is. But those babies just happened to be well-matched in temperament and came into the world with patterns close enough to the babywise system to adapt. OR the babies cried for what they needed until they gave up in despair and shut down. There are a number of websites talking about how terrible the author's dogma is and how dysfunctional his own family is.
While I have personally come to the understanding that you can't 'train' a baby to sleep any more than you can 'train' and adult, or a cat, to sleep, you can be sure the baby gets his needs met well enough to optimize his ability to relax and sleep. I could be wrong about this, but I've been watching young families for my 45 adult years, and that's how it looks.
You've received many excellent book recommendations, and I hope you'll read one or two and find something that fits your life and your baby's needs. This is an incredibly trying time for young moms, and double that if you are working. Yet somehow we live through it, and many moms even decide to have another baby as soon as they forget the intensity of the first few months.
I'm sure it would be an extra stretch financially, but if you could hire a mother's helper for an hour or three a day, or on weekends to allow you to catch up, you'd have your hands free for your 3yo, and perhaps time to catch a nap here or there. Even 20 minutes can be restorative.
Wearing the baby is a fabulous system for many moms. The baby relaxes and sleeps better when in close contact with Mommy as she moves around. It's somewhat like being in the womb. And sleep begets sleep, so the benefits could extend into the night hours.
There are some useful tips by Dr. Harvey Karp on getting babies into a relaxed state and enhancing their sleep. Look at these short videos and see whether they might help you:
How-to's: http://www.youtube.com/watch?v=G6KnVPUdEgQ&feature=re...
Enhanced sleep: http://www.youtube.com/watch?v=Tk5MUOMecHI&NR=1
Interview: http://www.youtube.com/watch?v=iu0TtxO-ocY&feature=re...
I wish your little family the best.