Well, this may be an unconventional answer, but depending on your educational background and experience -would it be possible to get a job now? I mean now before you have to tell anyone you're pregnant or would possibly know yourself? It DOES sound like maybe you need to have a job. Staying at home can be wonderful for the children, but if your husband is laid off and you're having financial difficulty, it's not a bad thing to do! If you're already several months along and/or showing -it's a pretty mute point unless you just try to get a seasonal job for the holidays. Even though it's illegal in some ways, it's extremely difficult to get hired when obviously pregnant because they know you're going to be out for at least 4-6 weeks.
So -birth control fails every day. If your MIL says anything, make sure you calmly look at her and ask her to google it. There is nothing short of a hysterectomy that COMPLETELY guarantees you'll never get pregnant. Even vasectomies, essure, tubals, etc., have between a 95 and 99% effectiveness rating -so sometimes something slips through! All you can do is tell your family that a baby is coming. The baby can't help it and all of you can try to be happy and enjoy the baby OR you can all be miserable and ugly and let the child feel that from the get-go. People who refuse to accept the unchangeable can also be put at a distance from your life.
As far as your husband -this is unfortunately a classic attitude when the frustration of finances, unemployment, etc. creep in. It's very hard for men who have never stayed at home to fully understand what a demanding job it IS to stay at home -and a thankless one. I would make sure since he's laid off that he's getting PLENTY of days -entire days -of caring for the kids on his own so he'll truly know what it's like.
As your pregnancy progresses, you will probably start to feel more joy and excitement even though you're in a stressful situation. If you don't, there's nothing wrong with talking about it to someone -your doctor -asking for a referral, your priest or preacher, a support group -etc. There are a number of free options out there. It would help your stress level to just talk to some people regularly and get those feelings out. You might even find a support group for people currently experiencing financial difficulty -that IS you!
Also -sit down with your husband and chart a REAL and realistic plan for the next year. Now that a new baby will be part of it, figure out what you BOTH need to do job-wise. If you go back to work, will you command enough to cover daycare expenses for 3 kids? When can your 4 year old enter a free PreK or kindergarten? How much is decent daycare/preschool in your area? A lot of this may show that it makes no sense for you to go back to work -or at least for one of you to stay home. Who has the best chance at getting the highest paying job? If it's YOU -then you go back and he can be the stay at home parent. Right now, you should get the house, financial picture and job situation back on track before anyone goes back to school.
Hopefully you still have some baby gear from the other two. The biggest expenses with infants are diapers and formula if you use it. You can get WIC for that if you need it to tide you over. If you need new baby gear -start looking on Craigslist and at Goodwill and Salvation Army centers. Go to garage sales. You can get TONS of great deals that way!
Part of your plan may include needing to rent a house or apartment for a bit until the finances pick up. You'll be able to find a rental easier than a home to buy with all the mortgage issues, etc. right now.
Make sure that part of your yearly plan includes a more permanent form of birth control after this baby arrives. He can get a vasectomy far more cheaply and easily than you can get a tubal or essure. If that's not a possibility, as soon as your post-partum is over, get a copper IUD. It's good for years and works immediately and very effectively.
Good luck!
***And I totally agree with Jane M. I used to escort at an abortion clinic, and many MANY women who came were just like you -not wild teenagers or 20-somethings who just wanted to party and have casual sex -they were women with families in horrible financial situations who felt way too stressed to deal with another child. It's the most personal decision a person can make, but thankfully in this nation we still CAN make it!