Biggest back seat area of Prius models???

i can only think the v (lowercase v for the Prius v wagon) which is the largest prius.

 

Prius vs Prius v (lowercase v for the Prius v wagon)

Rear Headroom 38.6/39.6
Read Hip Room 51.2/53.5
Rear Leg Room 36.0/35.9
Rear Shoulder Room 53.1/55.2
Cargo Volume 21.6/34.3

Obviously, the Prius v is marginally wider than a standard Prius and provides "more" room in the rear seat area. Only you can determine if it is enough of a difference in useable room, especially vs other cars. In your situation, I would definitely consider the Prius v over other Prius models.

I've never sat inside a Prius v but one obstacle you're going to face in a standard Prius that pure dimensions don't make apparent is the actual placement of 2 child seats and the remaining space for your 9-yr old. The standard Prius has a "hump" in the middle and it's difficult to properly place a child seat or booster seat there. Even placing them on either side presents its own set of problems. Because of the "cramped" space, our 8-yr old granddaughter had a lot of trouble buckling up in her booster seat, it was even hard for me. Fortunately, Toyota will provide seat belt extenders at no charge to make it easier and it actually puts the shoulder harness in a more appropriate location across the chest. If you go with a Prius, be sure to order the extenders. You will have to provide proof of ownership and sign a waiver. That's because the extender alters the final location of the harness and is basically designed for child seats and larger occupants. They don't want liability if a regular sized occupant uses it and the seat belt doesn't work as designed.

Also, be advised that 3 kids in that close proximity to each other is asking for trouble. Hopefully, you're not planning any long trips for a few years.

You can compare stats for all vehicles at the Toyota website:
Compare Toyota Prius VS Similar Competitor Vehicles

 

According to one member (whose name I don't recall) who complained recently about his vagon, it has less room for large child seats than a regular 3rd-generation Prius.

 

on the plus side, the v (lowercase v for the Prius v wagon) rear seat slides fore and aft.

 

Definitely not the Prius C.
I just got my Prius C (technology package-Canada)on Friday and am having a horrific time installing a single car seat in a rear facing position. I am using a seat that I didn't have any problems installing in a 2007 Honda Fit so I figured that it couldn't be too much of a difference in the Prius C. I couldn't be more wrong. I even had to consult with car seat specialists and the car seat company to see if I was doing something wrong. I was told by the car seat tech that the Prius is a difficult car for car seats. I don't know if they had considered the v (lowercase v for the Prius v wagon) in this assumption though. A good resource to check is a site called car seats for the littles.
I'm a bit of a car seat fanatic...

 

It's much cheaper to purchase car seats that fit your car than to purchase a car to fit your car seats. It seems most people jump to the latter solution first.

I have a 2012 PiP and we placed the toddler seat rear facing at first in the middle and the rear facing carrier behind the passenger. This was the optimal configuration as it uses up the empty space between the front passenger and driver. Also, the third passenger doesn't have to crawl around a car seat to get in. The spot behind the driver fits an adult. We've driven to San Diego (500 miles) with 5 people like this on multiple occassions. Toyota does not recommend using LATCH in the middle position so we use the seatbelt to secure the child seat.

For those who doubt the utility of a Prius C, there are car seats out there that fit three across rear facing. Some car seats better than others. Again, which would you rather pay for? $500 for 3 car seats or a brand new car?

 

In response to the last post, just to give perspective, the car seat I purchased is one of the narrowest, if not the narrowest car seats on the market (Clek Foonf) purchased for another subcompact vehicle. If it is difficult to fit a Foonf (albeit rear facing which adds to width) on the drivers side, it's going to be difficult for many. Best bet with whatever you buy, go to a shop once you purchase and ask to try some seats in your car.

I'm only jumping on this thread because I am in the middle of this dilemma right now. I'm by no means an expert at car seats, I'm at a loss. I am planning on getting a car seat tech to come help (or suggest a new seat). I will post again when that happens.

 

Agreed, it's not always about size. It's about fit. Being first time parents, we purchased an expensive $200 Recaro child seat rear facing in the middle position. It was a tight fit but we were happy with it. When it came time to switch to forward facing, the backside slant of the Recaro did not match the slant of the Prius backseat. I'm not a car seat expert. I did spend an hour in disgust trying to make it fit. In the end, we purchased a new Britax not a new car. We moved the Recaro to DW's car which fit perfectly forward facing in a Hyundai Sonata. It's quite a waste of money as I can count the number of times on my finger (1) last year that my daughter rode with DW.

In conclusion fit check all configurations of the car seat when you get it. Don't make my mistake.