Have you called your OB/GYN just to make sure there isn't a hormonal imbalance following the birth of your 3rd child that is exacerbating the shedding?
In my case, I really don't recall - when my daughter was 10 weeks old, I was diagnosed with cancer. 12 weeks into treatment, it was necessary for me to start wearing a wig publicly (at work, at least). By month 4 of treatment, I had only about 10% of my hair left, at month 4.5, it miraculously starting growing back despite still being on chemo.
I'm 16 months out from chemo, and my hair has started shedding again the past few weeks now that my hormones are stabilizing. I'm having to get used to having to clean dark hair off white counters daily (which I'd honestly forgotten I had to do).
My best advice, if you're concerned it's beyond normal postpartum shedding, it to make a call to your OB/GYN to see what they say.
Good luck!