I'm not an expert here.. but I would try to help
So you need an infant car seat for sure.. I think you can go in for Britax B-safe and install it behind the passenger seat.
Are your two grandchildren forward facing? I’m asking this because I know some people FF till 4 years of age even!
You could buy a combination seat for the 3 year old if she's not RFing.. a combination seat would help her because she would in any case be moving to a high-belt booster soon. A good recommendation for that would be Britax Frontier.. If you do not want to spend that much, you could go for Evenflo Maestro or Graco Nautilus. Nautilus and Frontier both have the same width.. but Frontier’s shell is bigger height wise..
The third seat can be a convertible.. Go for Radian or Recaro Pro-Ride because they both are pretty slim and usually work in the 3-car-seats-in-the-back scenario.. If you want to RF the GS.. you can install the convertible in the middle as Radian's/Pro-Ride's tall shell can eat into the driver’s space.
You can check out more details about these cars here:
www.britaxusa.com/car-seats/b-safe
recaropromotion.com/proRide.html
www.britaxusa.com/car-seats/frontier-85
I hope this info helped a little.. You could find more seats that fit your budget while meeting your safety criteria using this site.. www.carseatvs.com/dofind. You can even draw comparisons between two seats on this site..
Parents-in-Law. What do/did you call them?
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