Support a third serial port (#21784)

Marlin/src/HAL/STM32/HAL.h

@@ -68,6 +68,16 @@
   #endif
 #endif
 
+#ifdef SERIAL_PORT_3
+  #if SERIAL_PORT_3 == -1
+    #define MYSERIAL3 MSerial0
+  #elif WITHIN(SERIAL_PORT_3, 1, 6)
+    #define MYSERIAL3 MSERIAL(SERIAL_PORT_3)
+  #else
+    #error "SERIAL_PORT_3 must be from 1 to 6. You can also use -1 if the board supports Native USB."
+  #endif
+#endif
+
 #ifdef MMU2_SERIAL_PORT
   #if MMU2_SERIAL_PORT == -1
     #define MMU2_SERIAL MSerial0