We recently bought the 2008 Scion XB as our “family” car (for our not-yet-extant family). It’s very roomy inside - three adults can fit quite comfortably in the back. I don’t know about three carseats side-by-side, but that would depend on the carseat, as others have pointed out. It certainly looks plausible.
Other plusses are that it has a good safety rating; front, side, and side-curtain airbags; ample space for groceries, etc.; the reliability of Toyota innards; and it gets a little over 29mpg average (that’s roughly 60/40 city/highway, and that’s our actual, not the marketing numbers). And although it feels pretty enormous on the inside, it’s actually not as long as our Camry, and not as tall as your averge minivan/SUV, so it’s easy to maneuver, park, and get into/out of.
We paid under $18k for ours - you could likely get a used (or even new, if they still have 'em)one for considerably less.
I would advise you to only take advice from people who have actually done this. I know in our 02 Escape, with the seats we have chosen, you cannot fit 3 across in the back.
The problem with booster seats is you have to buckle the kid every time they get in the car. This can be very difficult if you’ve got 2 toddler or infant carseats next to the booster. I did this for a while on a Maxima - it was very difficult to physically get my hand in between the booster seat and the infant carseat to buckle the oldest child in.
Unfortunately, the best solution is a minivan or SUV. Having 3 carseats sucks.