drivers: Up to three units support

src/drivers/device/device.h

@@ -540,7 +540,8 @@ private:
 // class instance for primary driver of each class
 enum CLASS_DEVICE {
 	CLASS_DEVICE_PRIMARY=0,
-	CLASS_DEVICE_SECONDARY=1
+	CLASS_DEVICE_SECONDARY=1,
+	CLASS_DEVICE_TERTIARY=2
 };
 
 #endif /* _DEVICE_DEVICE_H */

src/drivers/drv_accel.h

@@ -83,6 +83,7 @@ struct accel_scale {
  */
 ORB_DECLARE(sensor_accel0);
 ORB_DECLARE(sensor_accel1);
+ORB_DECLARE(sensor_accel2);
 
 /*
  * ioctl() definitions

src/drivers/drv_gyro.h

@@ -83,6 +83,7 @@ struct gyro_scale {
  */
 ORB_DECLARE(sensor_gyro0);
 ORB_DECLARE(sensor_gyro1);
+ORB_DECLARE(sensor_gyro2);
 
 /*
  * ioctl() definitions

src/drivers/drv_mag.h

@@ -81,7 +81,7 @@ struct mag_scale {
  */
 ORB_DECLARE(sensor_mag0);
 ORB_DECLARE(sensor_mag1);
-
+ORB_DECLARE(sensor_mag2);
 
 /*
  * mag device types, for _device_id