Vehicles for 3 Carseats?

You might have to change car seats to make anything fit in a smaller car.
We also have a Toyota Sienna minivan. The only way to get 3 seats to fit in the same row is to use Diono/Sunshine Kids seats. When my third was born we had 2 in the high back Monterey boosters and the youngest in the Radian rear facing.
I can tell you that those 3 seats do not fit in the back seat of a Honda Civic which is what my husband drives.

I have a 2002 Camry and it holds 2 Diono/Sunshine Kids 5 point harness seats just fine. I am positive that a 3rd or an infant seat would fit also, but we only have 2 seats. My sister has a Mercury Sable, I'm not sure what year, but fairly new, and she could fit 3 carseats in her back seat with no problem at all. I think that if you want to go smaller, then you might have to go to a smaller footprint seat like the ones I have. If it makes you feel better, they are arguably the best carseat there is, but it will run you about $250 each unless you can find a deal or sale somewhere. They are worth it though, because they are also boosters so the kids won't ever need another carseat.

Good luck! buying a new to you vehicle is so difficult to decide on!

We loved our GMC Acadia and that fit 3 car seats comfortably. It had captain's chairs in the middle and a 3rd row. We just traded it for a suburban (obviously not the small SUV you're looking for!)

Why not wait to get a smaller family vehicle until your kids are older, maybe out of car seats all together. Then they can squish together in the back seat...lol.

Cars don't hold 3 car seats. Van's don't even do 3 car seats across.

What could work is putting your oldest in the back seat where they have the whole thing to themselves and put the 2 younger ones in the middle where they have easy access for someone helping them buckle.

Do you mean 2 booster seats? Or 2 5-point harness seats?

I can fit a rear-facing infant seat plus two boosters in my 4-door fusion (it's tight, but they fit). It would not work if it was 2 5-point harnesses plus an infant seat.

Think long-term, too. Eventually the infant seat will be a RF harnessed seat, then FF, and then booster.
So you need it to fit 3 boosters across to fit the kids until 10-12 (when they tend to 5-step to fit the regular seatbelt safely).

Get the widest you can find. :)
Really it depends a lot on the seats!

Cars and vans can do 3 across. We've done it for years.

I'm not in the US, but I have a Mazda CX7 which does the trick.