Why are there 3 wires on a brushless motor?
“Because there are 3 phases. Unlike a brushed DC motor, it doesn’t have a common ground.
On a BLDC motor the ground “shifts” between a pair of phases during rotation.
You have to remember that these 3 wires can’t be connected directly to a 2 wire battery.
Instead, the battery is connected to the (+) (-) leads on a complex electronic circuit called ESC which commutates the motor. The 3 wires of the motor connect to the output side of the ESC.“