I don't have my van any more, but when I did, it was a GMC Safari (huge, wide seats) with two bench seats. The third row was centered in the back, and the second row was the same width but slightly off-center (set towards the driver's side) so that you accessed the third row by scooting between the seats on the driver's side. IIRC, my third row was (driver to pass) RF TrueFit (32lb almost-4yo,) RF Radian (39lb just-turned-3yo,) Graco Airbooster (6yo.) My second row was Nautilus (not-quite-5yo,) MyRide (24lb 1yo) and SR22 (18lb 21mo... and yes, she still fit by height, Scenera (same kid) or Pronto (8yo.)
I had a loading procedure, essentially, kid in TF then kids in Radian/Nautilus would climb in. (Nauti kid would carefully step one foot in the MR to get over.) I would help MR & Scenera children in, then stand in front of the Airbooster to buckle the TF & RN kids. (They were old enough to put their arms in themselves.) I would move out of the way and the AB kid would climb in and buckle up while I buckled the MR child and checked the Nautilus child, and then I would check the AB child. Either I would buckle the Scenera child when I buckled the MR child, or the Pronto child would buckle and load while I checked the AB child, and then get checked.
Whew!
Until I turned my now-4yo, I would sometimes have five (!) RFers in my Pilot. (I got the Pilot after I crashed the Safari in Feb of '10.) I had a 3-across that was usually Radian (44lb not-quite-4yo,) MyRide (33lb not-quite-2yo,) and Avenue (33lb not-quite-5yo.) Then the second row would be Scenera (24lb 2.5yo) and SR35 (infant.) Whew! The 4yo has turned, so my third row is now RN/CCO/FFer, and the second row is booster/MR/SR35. I turned the 5yo when I turned the 4yo, but she wanted to RF again, lol.
Again, for me (with or without all of the RFers,) it's all about developing a loading procedure that works. It's a lot of trial and error for me. Over Spring Break, I only had two RFers, and wanted to change up my third row. We tried a totally new setup one day and it was a DISASTER. I had a 10yo and he had such a hard time getting out of his spot that he ended up sitting in a backwards MR trying to get out of the car, lol!!
So, load from the center, put the most self-sufficient kids in the hardest-to-reach spots, and choose easy-to-operate seats. Realize that you're going to have to climb, it's just part of life with a bunch of kids. I crouch up in my Pilot's tailgate daily. No skirts for me...
What van do you drive? General suggestion would be two and two, on the same side of the van (so pass/center or driver/center, both rows,) or load the third row kids through the hatch. Or, do your three youngest three-across the second row, and teach your oldest how to flip over the second row seat and buckle herself. Then you will just have to check her (via the hatch, I guess.)
It's do-able, just not the world's easiest thing.