🐛 Fix M575 port index output (#22553)

Marlin/src/gcode/config/M575.cpp

     case 2400: case 9600: case 19200: case 38400: case 57600:
     case 115200: case 250000: case 500000: case 1000000: {
       const int8_t port = parser.intval('P', -99);
-      const bool set0 = (port == -99 || port == 0);
-      if (set0) SERIAL_ECHO_MSG(" Serial ", '0', " baud rate set to ", baud);
+      const bool set1 = (port == -99 || port == 0);
+      if (set1) SERIAL_ECHO_MSG(" Serial ", AS_CHAR('0'), " baud rate set to ", baud);
       #if HAS_MULTI_SERIAL
-        const bool set1 = (port == -99 || port == 1);
-        if (set1) SERIAL_ECHO_MSG(" Serial ", '1', " baud rate set to ", baud);
+        const bool set2 = (port == -99 || port == 1);
+        if (set2) SERIAL_ECHO_MSG(" Serial ", AS_CHAR('1'), " baud rate set to ", baud);
+        #ifdef SERIAL_PORT_3
+          const bool set3 = (port == -99 || port == 2);
+          if (set3) SERIAL_ECHO_MSG(" Serial ", AS_CHAR('2'), " baud rate set to ", baud);
+        #endif
       #endif
 
       SERIAL_FLUSH();
 
-      if (set0) { MYSERIAL1.end(); MYSERIAL1.begin(baud); }
-
+      if (set1) { MYSERIAL1.end(); MYSERIAL1.begin(baud); }
       #if HAS_MULTI_SERIAL
-        if (set1) { MYSERIAL2.end(); MYSERIAL2.begin(baud); }
+        if (set2) { MYSERIAL2.end(); MYSERIAL2.begin(baud); }
+        #ifdef SERIAL_PORT_3
+          if (set3) { MYSERIAL3.end(); MYSERIAL3.begin(baud); }
+        #endif
       #endif
 
     } break;