My husband and I both commute to work by car, and our AP also needs to drive the kids to school and activities while she's on duty, so for us, a third car is a necessity.  However, we don't have a specific car that's "dedicated" as the AP car -- instead, she and I "share" two of our vehicles.  She drives our minivan during the week, while she has the kids.  That's the safest vehicle, and the one that the kids carseats/booster seats are in.  I have a little VW bug (convertible ;) ) that I drive to work every day.  Then during her free time, the AP uses the VW, and I use the minivan.  (My husband drives a big pickup truck that I don't feel comfortable driving, so we also don't let the AP drive it.)

I think a lot of it probably depends on whether your AP is expected to do any driving of the children during her work hours, and also whether you have decent public transportation available.  In our area, there's really NO public transportation, nor is there anything much within walking distance, so even if we didn't need our AP to drive the kids, we'd probably still have gotten an additional car for her.  (After our first AP who never "made a life" here and went home after 3 months due to extreme homesickness, we've learned that it's vitally important for an AP to make friends and have activities and a life outside of our home, so I think that having an AP who was "stuck" at home except when we could spare one of our cars would be a recipe for disaster.)