After doing my research, my personal recommendation would be to tell your grandparents on both sides that you already have everything you need and ask them for two Britax Frontiers for your two older kids. OR ask for gift cards to BabiesRUs Target and get it from them online. It will be the only carseat you'll use for both of them until they're 5. Ideally, #3 can stay in the ones you already have and move into 1's Frontier when he's ready for it.
Frontier is slimmer than any other Britax seat and will make it way easier for you to fit 3 carseats in one back row, if that's what you're thinking. If you get them on sale with free shipping, they're not much more than the second best ones and worth it because they'll last into the booster stage and you don't need a booster.
If you go Britax, the Frontier, then the Boulevard are the best.
Basically though, if your seats are installed safely and used correctly, then the kids are safe and that's what matters.
Here are the tips I got from the CHP when I had the boys' seats installed:
Make sure none of the belts or harnesses are twisted.
Always adjust the belts/head protection when your child grows.
The seat shouldn't move more than 1/2 inch in any direction. Sit in the seat when tightening the straps/seatbelt and use your weight to help you get them tighter.
Make sure your child isn't leaning back too far. They're not supposed to be lounging, the safest position is sitting up. Seatbelts/harnesses are designed to be used for up-right passengers.
The bottom/base of your carseat should parallel to the floor of your car.
The harnesses should be tight enough that you can't make a fold in the strap.
The chest lock should be placed across your child's sternum. NOT their stomach. THink about it- in a crash, do you want your child protected from whiplash by being held back up at their chest (where the ribcage/sternum will hold them) or do you want their internal organs crushed and their entire neck/upperback able to whip b/c the chest strap is too low.
SEcure any loose items in the car so taht they don't become projectiles in an accident.
Forget any after-market items like harness shoulder pads, piddle pads, mirrors, toys, sunshades as the carseats aren't designed to be used safely with them and they could become projectiles or compromise the safe use of the carseat.
Infant seats should not have the handle bar up. The CHP calls it the "Face Breaker" bar b/c kids often hit it in an accident and it can compromise the crush zone of the carseat.
There's my soapbox. =) My neighboy calls me the "CarSeat Nazi." =)
http://www.aap.org/family/Carseatguide.htm There's a chart of all seats, height limits and price at the bottom
http://www.nhtsa.gov/staticfiles/DOT/NHTSA/Communication%20&%20Consumer%20Information/Articles/CPS%20files/seats.pdf Ratings on safety of each brand/seat
http://www.car-safety.org/basics.html Some more info on individual seats
OR http://www.amazon.com/gp/feature.html?ie=UTF8&ref%5F=amb%5Flink%5F7724242%5F7&docId=1000290821 here's the link to some Britax close-outs that are cheaper than the second best other brands. the fabrics aren't bad either =)