I've only had 2 pregnancies, but I definitely showed a lot faster with my 2nd - some because my body had already stretched for the 1st, but also because I had a diastasis which is really common. Here's an excerpt:
"Pregnancy naturally separates a mother’s outermost abdominal muscles to make room for her growing uterus. Called a diastasis, this separation does not close up in most new moms. With each pregnancy this separation gets larger, which is why women show earlier with each pregnancy."
(taken from http://www.maternalfitness.com/education/lose_tummy_semin...)
Most OBs don't ever mention the diastasis to their patients, but a large majority of women have a diastasis of some size. I had the diastasis after my 1st (OB never mentioned it), but didn't know it until I was pregnant with my 2nd (self-diagnosed, confirmed when I specifically asked about it at my appointment!). So I ended up going to postnatal physical therapy to correct my diastasis (mine was 3-4 fingers' widths wide which I got down to 2 fingers' widths wide during therapy, and probably could get it down to 1 finger wide if I did my exercises with any regularity). But I never would have known about it from my OB - I had to figure it out on my own and then ask about it. A diastasis may not be what's happening in your case, but it's worth asking about. It's also easy to check yourself for one.
For more info, check out Julie Tupler's maternalfitness.com - she even has some good exercises for during pregancy:
http://www.maternalfitness.com/baks_basics.pdf
Congrats on your 3rd - hope it's an easy pregnancy that ends in a lovely, gentle birth!
http://pregnancytoday.com/reference/articles/childbirth.htm