I found that the 3rd seat is less useful than I was hoping it would be.

Basically, it's something nice to have for occasional use, but I can't recommend it as a permanent solution. For one thing, whith the 3rd seat in use, where will you put a stroller and all the other stuff? Even if it will fit somehow, you would have to strap everything down to make sure it doesn't end up on top of your kid. Pretty inconvenient. But that's not all. A rear-facing infant seat can't be installed on a rear facing seat. And the booster seat (Cosco) for our oldest says you should not use it on a rear facing seat either. I don't see why, but that's what it says. However, if you have a child that's big enough to ride without a booster, the safety belt will be positioned too high (across their neck) because of the way it's mounted. Booster seats with back rests have the belt guide which helps that, but manufacturer may say don't use it that way, see above - catch 22...
I assume the baby you are expecting is No.3 .. The trouble is one can't fit three child seats on the rear seat. All the child seats on the market are just too damn wide, at least from what I've seen.
About the safety factor, I saw some statement by a Volvo engineer saying it is very safe (can't find the weblink now). But I am not so sure what would happen in collision with an SUV.

Soooo, in my opinion, if you need to haul 3 or more kids, bite the bullet and buy a minivan.
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'97 850 n/a Wagon, 120K mi