I'd book the Yukon & beg for the upgrade... assuming they even have one on their lot when you get there. I'd also have a backup of 3 mid-size cars somewhere else (whatever company is the cheapest). When you get there, beg for the Suburban/Yukon XL and if not, tell them you're going to XX for 3 cars instead. If they can't get you into the big SUV, I'm sure they'd match your reservation at the other company if it means they can rent 3 cars.