Support GT2560 3rd extruder (#17797)

Marlin/src/pins/mega/pins_GT2560_V3.h

 //
 #define TEMP_0_PIN                            11  // Analog Input
 #define TEMP_1_PIN                             9  // Analog Input
-#define TEMP_2_PIN                             1  // Analog Input
+#define TEMP_2_PIN                             8  // Analog Input
 #define TEMP_BED_PIN                          10  // Analog Input
 
 //
 //
 #define HEATER_0_PIN                          10
 #define HEATER_1_PIN                           3
-#define HEATER_2_PIN                           1
+#define HEATER_2_PIN                           2
 #define HEATER_BED_PIN                         4
 #define FAN_PIN                                9
 #define FAN1_PIN                               8
 //
 #define SD_DETECT_PIN                         38
 #define SDSS                                  53
-#define LED_PIN                                6
+#define LED_PIN                               13  // Use 6 (case light) for external LED. 13 is internal (yellow) LED.
 #define PS_ON_PIN                             12
 #define SUICIDE_PIN                           54  // This pin must be enabled at boot to keep power flowing
 

Marlin/src/pins/mega/pins_GT2560_V3_A20.h

 #define LCD_PINS_D4                          21
 #define LCD_PINS_D7                           6
 
+#define SPEAKER  // The speaker can produce tones
+
 #if ENABLED(NEWPANEL)
   #define BTN_EN1                            16
   #define BTN_EN2                            17