3 bolts works fine, we're stopping bicycles not motorcycles here. The earliest for bicycle disc brakes of the modern (post-war) era used thread-on rotors for the most part until the mid-90s when bolt-on's took over because it made for faster/easier rotor changes, mainly on bikes being raced. When rockshox bought the amp brake design, they kept the 3 bolt format though in a different bolt circle diameter, even for the front DH setup on the boxxer forks (albeit with an undrilled 7.5" diameter rotor). If it was good for a 6" travel Pro DH bike in the late 90s, its good enough for most riders today.
Hayes didn't start with a clean slate of paper in making bicycle disc brakes like oh, Amp did. Instead they motorcycle disc brake designs, and most motorcycles had gone to bolt-on rotors with 6 bolts to hold them in place at the time.
When the biggest hub maker around (shimano) decided to settle on one design of bolt pattern, and went with the hayes pattern, it basically spelled the end for all other patterns unless the other disc brake manufacturers wanted to keep their customers locked to only a handful of available hub choices. That's why ChrisKing had their discotech hubs originally which used replaceable adapters to fit the different brands of disc rotors.
Cars which have a LOT of weight to deal with usually don't even have 6 bolts unless they're race cars. The disc rotor is usually held in place on most cars by the lug bolts that also hold the wheels in place. Why? Well if you happen to break the bolts, your wheel tends to come off first and that in most cases serves to stop the car just fine. The lack of a disc rotor kinda becomes moot then.