Do you have all the car seats? If so I’d suggest test-fitting them at a dealer to get a good feel for how it’ll work. Unfortunately the 8-passenger configuration is scarce, at least around here.

Currently I have a rear facing baby seat behind the driver seat and a high-back booster on the passenger side middle row. That barely leaves room for an adult in the middle of the 2nd row. It also makes the 3rd row only really accessible by sliding the passenger middle forward. Kids can do it but it’s hard to get adults in the 3rd row this way.

 

Wish I could be of more help, but here's what I know. We've regularly put 2 rear facing seats in the middle row (bench), 1 on either side, and that works well, although as said above, makes the middle seat of the middle row a bit of a tight fit for an adult, probably best if they're about 150pounds or less. And I've experimented with putting 1 rear facing in the 3rd row (driver's side) which was okay, but meant I had to move the middle row up, and therefore my seat up a bit more than I like. And we're expecting a grand baby in June, so we'll probably do 2 in the middle row (either side) and 1 in the 3rd row whenever the need arises. Haven't tried to carseats side by side in the middle row yet, but depending on how narrow the carseats, I would think if you did driver's side and middle, the right side should maybe still be able to fold down. And pretty much every scenario we've tried, or thought about, does have a bit of a downside. When we've got 1 on either side of the middle row, it's more roomy for a teen or adult to sit in the 3rd row. They can crawl in through the back hatch, which isn't too bad, but a bit of a pain, or they can squeeze behind the 2nd row, but to do that we have to first move the first row passenger get out and move that seat all the way forward so the 2nd row can go all the way forward, so that there's room enough to get to the 3rd row that way. Sadly, carseats, for all their wonderfulness and safety, can be a huge pain in the butt and take up a decent amount of room in a car. I'm definitely looking forward to the day I no longer have any rear facing seats in the car to deal with. Anyway, hope that helps a little bit, and let me know if you have any follow up questions. And feel free to ask me again in June when we've tried the 3 seat configuration.

 

You should be able to if you use Diono car seats, they specifically advertise this capability (all 3 on a bench) with one particular slim model.

 

I’ve got a captains chairs with a rear facing on the passenger side middle, and two forward facing on the driver side middle and 3rd row. I usually keep the other 3rd row seat folded down for extra cargo room since you’re not fitting a full load of groceries in the back with both up. It’s a tight fit to put anyone in the other 3rd row, but you can fit a smaller adult there if you slide the front passenger seat up pretty far. It’ll fit a lot more comfortably in another 6 months when we’re done with rear facing.
If you go bench, I can confirm the Dionos work. Use them in our Infiniti suv with 3 across.

 

I usually keep the other 3rd row seat folded down for extra cargo room since you’re not fitting a full load of groceries in the back with both up… I can confirm the Dionos work. Use them in our Infiniti suv with 3 across.

How many people are you buying groceries for?? Maybe it’s because I pack the bags myself after shopping at Aldi, but I can fit at least 4 of their big reusable grocery bags in the trunk with the back seat up. That said, I rarely use more than 3 of those bags at a time shopping for a family of 7

I haven’t used diono’s in the Ascent but back in the day I did fit 3 of them across the back seat of a Chevy Cruze!

 

Pick your poison on this decision! But certainly test fit if you have the car seats already

There have been a lot of conversations how people like the bucket 2nd row for ease of 3rd row access of car seats. Very valid....but....

Car seats are short lived in the grand scheme of the childs life. Once they are grown out of the car seats the bench seat may prove more valuable for "easy in, easy out" with 3 kids in the 2nd row. If you go with bench seat then you yield a LOT of room in the rear by folding down the 3rd row.

So while you re looking for a near term answer, dont forget to consider how long you plan/want to keep the car.

 

We only have two car seats, a rear facing convertible and a forward facing booster, and originally looked at a second row bench. After seeing it in person, we decided it wouldn't work for us and the third row would be too difficult for us to access. The forward facing booster is in the passenger side captains chair to make school drop off and pick up easier and safer. The captains chair is all the way back and the car seat is installed using the seat belt, so we get to the third row by going in front of that seat and down the middle. Easy for kids and smaller adults. The rear facing convertible is installed in the captains chair behind the driver using the latch. We can't really get to the third row from that side.

We looked at this write up when we were deciding: Subaru Ascent - The Car Seat Lady

 

Now that I have a few minutes to search, this is the Diono seat capable of 3 across. Solid build using a sturdy frame not plastics. (I have Ranier model - not the 3 row one in the video, for my 3yr old)

Here's a mom vlog with her Ascent car seat tips:

 

We weighed getting the bench seat middle with 2 car seats and ended up with the captions chairs because it was easier to get to the 3rd row with them. The pass through isn't huge but enough for an adult to use. Also, in the future they would be easier to use with boosters.

We have Diono car seats, one Radian one Pacifica. For a lot of 3 wide car seat use I wouldn't bank on getting to the 3rd row easily. Most times we put my oldest in her booster in the 3rd row for easier access to seating.