J.,
You seem to have a lot on your plate. Three kids and all. Do you have help? If not, don't presume that you should be able to simply "handle it." In my observational experience, most women with 3 kids are close to the edge much of the time. Learn to ask for help.
As far as PPD goes, hormone imbalance is also likely a culprit. Most women these days have hormone imbalance, which is largely due to poor nutrition, lack of sunshine, lack of sleep, stress of life, lack of exercise, too many toxins in the body, etc. For an official and competent evaluation, you might try Dr. Margaret Christensen www.christensencenter.com. If you can't get an appt, ask to be put on the waiting list.
If you go to your conventional ob/gyn, he or she will likely just prescribe drugs. This may be a short term option, though drugs are not intended or designed to actually heal anybody in the long run. Side effects are not to be underestimated. See www.mercola.com for more info on side effects of these psychotropic meds. Still, you may feel you need them short term. If you try to get healthier by addressing lifestyle issues mentioned above, however, you will feel much better over the long term.
Good luck!