3 boosters across? Or is this necessary?

[MLB]

I've got an 07 Ford Freestar. The back row is extremely tight (45 inches) and I need to fit 3 kids across there. This isn't going to be necessary for a year but if I need to buy a new vehicle, I need to start planning. When this becomes a necessity my family will look like this:

2.5 year old - Graco Nautilus, middle row
5 year old - Safety First Alpha Omega, middle row
7 year old - ?????????????? - back row (he will probably be close to 60 pounds by then)
10 year old - ?????????????? - back row (weight unknown)
12 year old - ?????????????? - back row (weight unknown)

We are adopting the 10 and 12 year old, therefore we have no idea what their weight is. I would like to put them in a backless booster but I'm not finding any way to make that work.

I'm thinking about putting the 7 year old old in a high back booster and having the 2 older kids go without boosters. Would this be a mistake? I'm sure they aren't yet 80 pounds.

Does a booster seat need to be fully supported by the seat below? I was thinking I could fit three of the Graco Turboboosters and the arm rests and maybe a small portion of the seat itself would hang over the edge of the bench seat. Would this compromise safety?

I was also considering the travel harness but I see no one recommends using them as your main child restraint system. In this situation, with 2 older children, would that be reasonable? If we did this I'd put the 7-year-old in a Graco Turbobooster then have the two older kids in travel harnesses. I assume this would be safer than having these two kids without boosters.

Any suggestions on what would work best?

[MissKatie]

Depending on height, the 12 year old may be able to ride without a booster. It's generally around that age that kids start to pass the five step test. If that's the case, I would put the ten year old and the seven year old in the third row outboard seats in boosters, and the 12 year old in the center without one (as long as there is a center headrest. Otherwise you would need a child in a high back booster or a harnessed seat there).

My only other suggestion would be to switch the five year old and the two year old's seats. Most five year olds have outgrown or are very close to outgrowing the alpha omegas (keep in mind that most of the models with the red tabs to adjust the harness cannot be used with the harness in the highest strap position). That child would probably be better in the nautilus with the younger, smaller child in the alpha omega.

[atsuydam]

Nope, you can't fit three booster back there, unless you have very long agile fingers and don't care what you knuckles look like :D I was thinking you might be able to Turbo booster-radian-Turbobooster but I can't get my hand down to buckle it up.
But I put my daughter in the seat with a booster. Her feet reach the ground and if her legs were a bit longer she would fit:
http://i563.photobucket.com/albums/ss79/atsuydam/Bethanynobooster.jpg
So unless the 12 yo is incredibly small (DD1 is 48", 54# @5y8m) they should pass the five step back there. I would put the 12 yo on the passenger side, booster, then a Radian for the 2 or 5 year in the driver side third row. I don't like how the center belt comes down from the ceiling. The little seat belt adjuster they gave for it is kind of useless.
Then I would do one booster in the middle row and the other forward facing car seat.

BTW: the booster was only like that for my experimentation, DD1 rides in a Regent at her age, with occasional rides in the TB with back ON! :) )