WHat age car are you planning to get? Ours is 12 years old and still going strong (the big boxy pre 2000 model). Has done 120K, they're meant to be good for up to 200, so I'm hoping for another five years+ [fingers crossed].
I've never put three car seats across the back, but you certainly could in ours as long as you used the built-in booster seat in the middle. Be warned - the seatbelts on volvos are relatively short, so you may not be able to fit a rear-facing child seat. I couldn't in ours, anyway, had to switch to a forward-facing one.
The old ones have the extra fold-down seats in the boot, which are very useful but I think no longer provided in the newer models.
Otherwise? Great car, not very fuel-efficient, particularly on shorter journeys, though better on motorways. Ours is a petrol model, diesel more economical if you're doing higher mileage. Feels reassuringly solid and is nice to drive. Other cars do get out of your way when they see you coming up from behind, which is not true ime with smaller more girly cars. The newer Volvos, ie after the Ford takeover, are significantly less solid than the older Swedish ones, which might put me off.
HOWEVER: if I was buying now, I would seriously consider getting something more fuel-efficient, which would also have lower road tax and lower rate for residents' parking (London). I'd be looking at a Skoda Octavia, perhaps, or maybe a Prius if they ever bring out a decent family-sized one with a proper boot.
hth