I'm glad you're talking about it. ITA about the fact that none of us were cut out to raise children in isolation in a non-child-friendly culture. It's way harder than most of us let on a lot of the time. Keep talking, mama.
I did want to say, though, that feeling like your kids would be better off without you is a CLASSIC symptom of PPD. I know your youngest is 8.5 months old, but PPD can have onset up to a year (maybe 18 months?) after birth. I know that with multiples, it often happens later, I think because after the birth you're in such a crisis mode, and I wonder if that might not be the case with 3 little ones too. Please get in touch with your HCP (midwife, OB, family doctor, nurse practitioner, etc.) and ask to be evaluated for PPD. There is a lot you can do, from talk therapy to cognitive-behavioral techniques to medication (if you choose). You need support and care and down-time, but you may also need to help your brain get back to normal. You can't just *think* yourself out of depression, though people like to believe that you can. That's like trying to *think* yourself out of diabetes. You can change your eating habits or your thinking/feeling habits, or you can take medication to help, but it won't just go away because you want it to.
Here is a self-assessment quiz that might give you some insight: http://www.kellymom.com/ppd/ppd-quiz.php
and a list of resources about PPD: http://www.4woman.gov/Pregnancy/pg.cfm?page=275
. Here is another self-assessment: http://www.pndsa.co.za/ms-fc.htm
This one categorizes "feeling your baby would be better off without you" as being an emergency symptom that indicates you should call your HCP right away.
Hang in there. Many amazing women and moms I know have dealt with PPD. You can get through this and you can enjoy parenting and your life (not every minute, but all together).