I suspect that the car quote will not have taxes included, which in Europe are very high, but I could be wrong. Also check autoeurope.com. They will give you a quote that includes everything so you know exactly what it will cost. They are broker, not a direct leasing agency. I've used them several times and been satisfied with what I got.
For three people, having a car for that price for three days is a reasonable cost (per person per day) in my opinion. And no matter how good public transportation is, a car is usually much faster and gives your more time to enjoy the travel experience. I'm a fan of car rentals in the country, but definitely not in the larger cities.
In any event, do let us know how this works out for you. Bon voyage!