I don't have a Mazda 3, but I have a hitch-mounted rack. I have a Thule T2. Loading and unloading is a snap. If you got an after-market receiver hitch (you'd have to get a 2"), you could carry up to four bikes on it.
Pros:
Easy to load and unload
Very stable
You can access your hatch when using it (even when bikes are on the rack).
Cons:
Expensive
Not convenient to take it on and off the car, so it will generally live there forever
Possible rear-end accidents make me sweat