Hellobee Boards

For safety reasons it's best to put the most vulnerable child in the middle, in your car the forward facing child.
You're right that you can't use the latch anchors for two seats, it compromises the weight they can hold.
Using seatbelt installs will save you some space too.

@PinkElephant: Yea I think our seat was 20lbs and DD is 40 so it was kinda a no brainer to instal hers with the seat belt.

@PinkElephant: I have a van so I actually have the baby in the third row.

You can only use latch for the center seat if the car manual states latch borrowing is allowed (most of the time it isn't). Typically if you have latch on the two outboard seats those are the only places latch can be used. you can't "borrow" and latch install in the center (check your car manual)

I would suggest posting to car seats for the littles Facebook page. They are awesome about troubleshooting things like this. They have a lot of CPSTs who moderate the board (make sure you read the user agreement first).
They will probably ask if any of the belts overlap on your vehicle and if you can get safe and independently tight installs of all seats. They can most certainly suggest the best option for the new babe, or if any of the kiddos need to be shuffled around in their current seats.

@PinkElephant: that's the beauty of the van. I can use the third row and still have trunk space. I can also sit between my big girls to make sure they're behaving.

I know this isn't specifically a post about seats, but the new Click Tight technology on the Britax seats is really great for a painless seatbelt install. We have the Frontier and it's so easy to install with the seatbelt.