3 DC motor sketch error, simple, i thought?

[TomGeorge]

Hi,
I think you need to do this in assigning your motors

AF_DCMotor left_motor(1, MOTOR12_1KHZ);
AF_DCMotor right_motor(2, MOTOR12_1KHZ);
AF_DCMotor backleft_motor(3, MOTOR34_1KHZ);

AFMotor Library Reference

Tom...