One suggestion I would have is to take all of your car seats and/or booster seats and put them into the positions you would normally have them, or would like to have them (Especially since you will have a rear facing seat shortly - Congratulations!!). Then adjust the front passenger seats where you would want them and see if everything works out. I don't think the Explorer will be able to fit 3 across in the second row with a bench. And know that the 3rd row in the Ex only has latch on the passenger side. Also know that the 2nd row bench on the Ex is not adjustable fore and aft. It might be just about impossible to get passengers into the 3rd row with car seats in the 2nd row without removing them first (I am not familiar with how the seat can move with a car seat in it).
We have a 3yo and his seat is on the passenger side 2nd row. We also have the buckets with a console, so that leaves us a free seat to tumble to allow access to the 3rd row. If we are carrying 6 people around regularly (not a common occurrence, but happens when we are toting out-of-towners around), we will move him to the 3rd row, which he actually seems to like better.
Also, you can get the buckets without the console, which would allow for 3rd row access, but I would hesitate before doing that. The 2nd row buckets do not have built-in armrests, so your inboard arm has no place to go. That being said, if those two seats are going to be occupied by car seats for the next couple of years, by the time that matters, it might be time for a new car anyway
Taking that into account, the 2nd row buckets with no console might be the best, if not possibly the only, way to get everything to work.
The Ex is a great vehicle and we love ours, and I would highly recommend it. One other thing you might want to check is how well all the strollers fit in the back with all of the seats up and ready for use.
Hope that helps