since you're not able to buy new seats, but the seats you currently own will make it difficult to do 3 across in most vehicles (and I saw that you are looking into 3 row vehicles, but with a tight budget you may find a better quality car vs. suv or van, I've been there), perhaps you could contact your local police station, or safekids network, maybe talk to a social worker at your kid's doctor, there are programs available to help low income families get appropriate car seats at little to no cost. I understand that you have a lot to think about, so maybe being able to get assistance with car seats would give you 1 less thing to worry about? I also think financially it makes more sense to go straight to a convertible car seat from birth, for $40 or $50 you can get the baby a seat that will last for 2 years, maybe longer (specifically thinking about a cosco scenera or evenflo tribute, and they are fairly narrow), you can than wear the baby (which leaves 2 free hands for the twins) or use a stroller. if you really want an infant seat (they are convenient at times) the baby trend flexloc is the narrowest seat at a lower cost, I believe, if you are able to get a 3 row vehicle and don't have to worry about space the evenflo embrace 35 at walmart is $80 and long lasting, also the graco snugride 30 0r 35, or safety 1st onside air are long lasting infant seats). If you are able to get new seats for the twins you can always sell the chase seats they're in to get back some of the money you spent.
I also agree with an odyssey, pilot, impala or taurus as far as vehicles