Support a third serial port (#21784)

Marlin/src/HAL/DUE/HAL.h

@@ -68,6 +68,16 @@ extern DefaultSerial4 MSerial3;
   #endif
 #endif
 
+#ifdef SERIAL_PORT_3
+  #if SERIAL_PORT_3 == -1 || ENABLED(EMERGENCY_PARSER)
+    #define MYSERIAL3 customizedSerial3
+  #elif WITHIN(SERIAL_PORT_3, 0, 3)
+    #define MYSERIAL3 MSERIAL(SERIAL_PORT_3)
+  #else
+    #error "SERIAL_PORT_3 must be from 0 to 3, or -1 for USB Serial."
+  #endif
+#endif
+
 #ifdef MMU2_SERIAL_PORT
   #if WITHIN(MMU2_SERIAL_PORT, 0, 3)
     #define MMU2_SERIAL MSERIAL(MMU2_SERIAL_PORT)

Marlin/src/HAL/DUE/MarlinSerial.cpp

@@ -486,4 +486,9 @@ void MarlinSerial<Cfg>::flushTX() {
   MSerialT2 customizedSerial2(MarlinSerialCfg<SERIAL_PORT_2>::EMERGENCYPARSER);
 #endif
 
+#if defined(SERIAL_PORT_3) && SERIAL_PORT_3 >= 0
+  template class MarlinSerial< MarlinSerialCfg<SERIAL_PORT_3> >;
+  MSerialT3 customizedSerial3(MarlinSerialCfg<SERIAL_PORT_3>::EMERGENCYPARSER);
+#endif
+
 #endif // ARDUINO_ARCH_SAM

Marlin/src/HAL/DUE/MarlinSerial.h

@@ -149,3 +149,8 @@ struct MarlinSerialCfg {
   typedef Serial1Class< MarlinSerial< MarlinSerialCfg<SERIAL_PORT_2> > > MSerialT2;
   extern MSerialT2 customizedSerial2;
 #endif
+
+#if defined(SERIAL_PORT_3) && SERIAL_PORT_3 >= 0
+  typedef Serial1Class< MarlinSerial< MarlinSerialCfg<SERIAL_PORT_3> > > MSerialT3;
+  extern MSerialT3 customizedSerial3;
+#endif

Marlin/src/HAL/DUE/MarlinSerialUSB.cpp

@@ -134,6 +134,9 @@ size_t MarlinSerialUSB::write(const uint8_t c) {
 #if SERIAL_PORT_2 == -1
   MSerialT2 customizedSerial2(TERN0(EMERGENCY_PARSER, true));
 #endif
+#if SERIAL_PORT_3 == -1
+  MSerialT3 customizedSerial3(TERN0(EMERGENCY_PARSER, true));
+#endif
 
 #endif // HAS_USB_SERIAL
 #endif // ARDUINO_ARCH_SAM

Marlin/src/HAL/DUE/MarlinSerialUSB.h

@@ -59,3 +59,7 @@ struct MarlinSerialUSB {
   extern MSerialT2 customizedSerial2;
 #endif
 
+#if SERIAL_PORT_3 == -1
+  typedef Serial1Class<MarlinSerialUSB> MSerialT3;
+  extern MSerialT3 customizedSerial3;
+#endif