The reason it’s the fastest is mainly due to its unique engines, find out how they work here:
The SR-71 Blackbird was designed to cruise at Mach 3+, more than three times the speed of sound at altitudes up to 85,000 feet. Because of the unique capabilities of the SR-71, it made the perfect test subject. NASA used it to test aerodynamics, propulsion, structures, thermal protection materials, high-speed and high-temperature instrumentation, atmospheric studies, and sonic boom characterization. NASA conducted tests intermittently using the SR-71 until 1997.