Only thing I can say is measure the row & the seats you are keeping and then try to find another seat that fits in want space you have left. We have 5 kids in seats... 7 mo old in a carrier car seat, 22 mo old in a 5-point harness and 3 in boosters 5 yrs, 7 yrs & 8 yrs. Make sure you follow your states laws... our laws in Ohio is rear facing till 1 or 20#, 5 point harness seat till 4 or 40# and booster till 8 or 80# & 4ft 9 in. My daughter is 8, but she is only about 70# - so she is still not able to be out of the booster. Also, I did call out local hospital just before I had the baby... booster seats are not made for the middle lap belt only - they are made for the shoulder belt & actually if you are using only a lap belt it is safer not to have a booster, but it is agaist the law not to use the booster.
We have one booster in the middle row & the youngest in the rear facing sear, the we have the boys in the 3rd row... booster on each side w/ the harness seat in the middle because it is only a lap belt. But we have a 7 seater mini van... also I though air bags were a no no for kids - are the side air bags different from the front air bags?
Good luck figuring it out, but honestly I think your 3rd row is a waist if you can't even put your kids in it - I know not to buy an Explorer when we get a new can.