The 3V motor is series wound with brushes. This means that the current from the battery goes through the field coils, making a stationary field then the current goes through the armature via the brushes, causing the armature to rotate. As the motor is rated at 1800 RPM, it will not generate much voltage until the armature is spinning at this speed and it also needs a load on it because, no load, no current, no magnetic field, no generated voltage.
Frank
Get a fixed field (magnets!) brushed motor, it will work better.