I've noticed this, too. I've been shopping for weather-proof floormats and haven't seen anyone offer a mat for the 3rd row. I figure the thinner, more flexible mat would be easier to trim down for the 3rd row if needed. You say it "will be a family hauler", definitely go check out one at a dealer and get a good feel for what the third row floor looks like so you can shop around better. Good luck!
On another note, the 3rd row for me will not be in use and is going to be constantly folded for cargo room. I found this online and thought it'd be a good idea for the back seat and to put under a cargo tray mat for added protection:
2013-2014 DODGE DURANGO Canvasback Cargo Liner (Black) [2nd row Captains seating][Covers back of 2nd and 3rd row seats and cargo area] : Amazon.com : Automotive
Hey all. 2015 durango For me, I have the captain chairs in second row. When I ordered it from weather tech i had to make sure to get the three row with captain chair option. All 3 rows fit perfectly. I have noticed on the site they don't have anything that fits custom for cargo in the rear but they do make 3 row that fits great for captain chair 2nd row with storage center.