Hi,
Congratulations on the upcoming birth of your third child, who will have older sibs. That is a great family!
I think you will enjoy a mini van. They can be luxurious and are very comfortable.
I am a sometimes kid-hauling grandparent, and DH and I drive a minivan. Because we did not want to have car seats in our car at all times, we got built-in booster seats. They have harnesses and also work with shoulder straps. They flip out of the way for adults to sit there.
Our seats are approved for babies over 22 pounds. They are safe, but they do not lift the baby up very high. So a baby cannot see out. They work very well for older kids and for occaisional use.
We did have one built-in booster seat, now we have two. We have generally had Chrysler products, currently a Dodge Ram mini-van. DH and I find the mini-van more comfortable than my luxury Buick sedan.
The one thing I would recommend is that you try a few different brands. Some of them are easier to see out of than others. I can see out of the Dodge better than I could our Chrysler Town and Country, my DH could see out of the Chrysler better. So that definitely is personal preference and probably depends on your height.
We also have stow and go seats in the rear, which is nice for cargo, luggage, etc.
And I personally think a two-seat bench in the middle is better because you can get into the back more readily to tend to that third child.
HTH
Best wishes,
E.