Congrats Cybear!
You can put a car seat against each passenger door and put the oldest child in the middle.
With my job, I have to transport kids pretty regularly (and have two kids of my own), and that's how I usually do it. It is cramped, but doable. Not sure I'd travel for any great length like that though (causes GKS...Grumpy Kids Syndrome).
I agree with the others though, I wouldn't add a third row if it's not intended to be that way. You may unfortunately have to sell the X and get a...God forgive me...Caravan. Or Expedition, which isn't too bad.
Sacrifice...that's what being a good parent is all about huh?
Now, as Dr. Laura says, "Cybear, go do the right thing."