IME, it is difficult to get 3 seats across in a Subaru, but not impossible -- 4 years is the bare minimum to "graduate" into a
booster (each graduation is a decrease in safety) & it's often easier to buckle harness seats in 3 across situations ... once they've been installed: each must be
independently secure with less than 1 in. movement
at the belt path.
Can you please list each child's actual age, weight, height?
It's hard to help with just a description of "huge"

Also, what is the actual vehicle year, make, model?
I have not personally installed any seats in a Passat, but I heard good things when we were considering that vehicle

I'll ask around some more, but have you also seen the Mazda 5? I think that's it

Sized like a small station wagon, 3 rows of seats, 2 seats in each row.
At the very least, IMO, your 4 year old should have a highback
booster as research shows they are safer in side impacts, especially those that have deep head wings filled with EPS/EPP foam (
Britax Parkway tends to fit kids of all sizes pretty well, adjusts very high, is reasonably priced & the lack of armrests make it easier to use in 3 across situations).
I think I remember it being possible to have
Marathon between
Snugride or
Radian &
Parkway in a Subaru.