A couple thoughts, based on working three across in DH's 2006 Accord....
1. You most likely will not be able to use a backless booster in the center as there is no headrest there. If you put a booster center, it must be a highback if there is no headrest.
2. Buckling a booster center in a three-across is painful at best and impossible at worst. You also risk the booster child unbuckling the other seats.
I think I would look at getting a Radian for your DS2 and putting that center. That will give you the most room. Since you have lower anchors, you can use them to aid the center install (you can't install with them, but you use them to get it tight and then unhook them after the seatbelt is tight). Then put the SafeSeat outboard on the side where the center seat buckles (probably driver's side) and a highback booster outboard on the other side. You will have to use the seatbelt to install the SafeSeat, but that isn't a big deal. For the booster, I would look at something with decent side impact protection, but not huge, like the Recaro Vivo, or hopefully this summer the new Parkway will be out as well as the Chicco Strada - in your case I would really wait to see if those two come out by the end of summer. When the SafeSeat is outgrown, you can see if the Fisher Price seat will fit next to the Radian RF, or possibly get something narrower if it doesn't.
Mama to three boys ('03, '05, '07)