You have got to talk to your OB and get a referral to a psychiatrist (can RX meds) or a psychologist (can not RX meds). It's not something that you can ignore or just get over. It can get worse if you don't get yourself treated at the very least with therapy.
Things that can help:
1. Be sure to shower and get dressed every day.
2. Try to get out of the house once a day or every other day.
3. If you can get out alone a couple of times a week, do it, even if it's just for a walk or running an errand while someone else stays home with the kids.
4. Be sure you have someone you can call when you need it most. Sometimes just having a sympathetic ear to vent to can help.
5. If you feel overwhelmed and the kids are crying or giving you a hard time, make sure they're safe in their room, put the baby in the crib or play yard, and lock yourself in your room or sit on your porch with a big glass of your favorite drink. Sip your tea or lemonade or whatever slowly. Do breathing exercises like you did during labor. Do not move from that quiet spot until you've finished your drink completely, even if the kids are calling. They can handle being without you for a few minutes, especially if it means you're calming yourself down.
6. If you can get some exercise, that can help your mood. It releases endorphines and naturally boosts your mood.
7. Nap when the kids are napping. I don't care if you have "one more load of laundry." If you're not rested enough, you're more prone to depression and getting sick as well as gaining weight.
8. Keep up with your vitamins, especially B complex vitamins.
9. Don't be afraid to allow your psychiatrist to prescribe an anti-depression or anti-anxiety medication. Sometimes, some of us really do need the additional help and there's nothing wrong with that.
As you can tell, I've been there. I didn't have my PPD properly treated after dd#1 or #2, but finally after dd#3 weaned herself from breastfeeding at almost 6 months old, my PPD kicked into high gear. She's going to be 2yrs old next month and I'm still being treated. It started as treatment for PPD but soon became treatment for other underlying anxiety and depression and OCD issues. My only regret is that I didn't do it sooner. You can private message me if you want to talk more.