Brushless DC motor - number of hall sensors
You need all three hall sensors.
You can infer the third phase by measuring the other two phases in a 3-phase sinusoid but that is because there are continuously varying so you can distinguish exact points in the cycle based on value, but the square wave of hall encoder outputs have flats (since it is a rectangular wave) so you cannot distinguish where the third phase is in the cycle by using the other two phases.