Booster Seat for 3 Across??

We just moved my 6-year-old to a booster b/c he outgrew his harness (bought a Clek Oobr), but it will be difficult to buckle anything with three across for sure. We have 3 across, too. RF seats for the 1 and 3-year-old, and then the booster for the 6-year-old, but it's a van so easy.

I like the Oobr as it should fit him until at least age 10, so it could be his last seat potentially. I don't plan to take the back off unless he outgrows the shoulder guide by height. It is over $200 so pricey, but it's a great seat with structural support and good side-impact protection.

I like clek olli brand.. Super comfortable, clicks into latch system, durable safe... Grows well with your child. We have two of them.. One seat is 4 years old and the seat looks almost brand new.
http://clekinc.com/olli/

Sunshine Kids which is now Diono make a wonderful high back booster that attaches with LATCH. We had 3 across of the Sunshine kids, 2 boosters and one rear facing 5 pt harness when my third was born. My oldest is now big enough to not need a booster but my just about to turn 9 year old is still in his booster and my youngest is in her 5 pt front facing now.
http://us.diono.com/en/car-seats

Safety First Go was recommended to me for that reason. My 7 year-old still rides in it. It's very thin. It HAS to be installed with a top tether hook, though, because it folds up into a bag for storage. It's an awesome travel seat too!