I did three across in a Civic when I did some childcare and found it really miserable. Still have the Civic but now the kids are out of carseats and it is still miserable! There just isn't much shoulder room. Contrasting that to my father's Accord, and there is plenty of room (but I'd still never do a road trip with 3 across). So I think it depends mostly on the width of the backseat, as to whether it is tolerable or miserable.
You can rent a van for road trips, maybe (in many areas they aren't available or cost a fortune so check it out in advance). Think about how often you want to drive a few hours or go camping. With five people (even tiny people, even packing minimally) you just don't have a ton of space until you go to a minivan. For us, with two kids, we needed more space to camp, period.