Is your 2nd child old enough to be in the front seat? If so any car should be fine. When your oldest is home, the 3 little ones can squeeze in the back.
Not all back seats are created equal. I find the backseat of the Toyota Yaris to be more spacious then some bigger sedans for example. We loved having an SUV but decided that for the rare occasion that you actually need all that extra space, it would be cheaper to rent. A car is cheaper to buy and to fuel.