Which do you think is cheaper for travelling: auto, taxi, or bus?

It depends upon how much you travel. For a single trip across town a bus may be the cheapest. However, if you are making multiple trips a car is probably less expensive. It really depends upon how often you travel and what the convenience of being able to go anywhere at any time is worth to you. Cars are great for convenience. Another factor is the type of city in which you live. In Tucson, AZ where I live bus and taxi service is very limited and parking (mostly free parking) is abundant so a car is the perfect means of transportation for most people. However, in places like New York City, Moscow, London, etc. where the streets are jammed, parking limited and expensive while busses and taxis are abundant and cheap, then a bus is your best bet. While I will drive through a city like New York, Paris, etc. I refuse to drive to destinations within a city. When I find myself traveling by car to cities such as these I generally go straight to my hotel or friends where I am staying and leave my car parked there until I leave town. In town I use the bus.