Rename CORE_AXIS_3 to NORMAL_AXIS

Marlin/Conditionals.h

   #if ENABLED(COREXY)
     #define CORE_AXIS_1 A_AXIS // XY from A + B
     #define CORE_AXIS_2 B_AXIS
-    #define CORE_AXIS_3 Z_AXIS // normal axis
+    #define NORMAL_AXIS Z_AXIS
   #elif ENABLED(COREXZ)
     #define CORE_AXIS_1 A_AXIS // XZ from A + C
     #define CORE_AXIS_2 C_AXIS
-    #define CORE_AXIS_3 Y_AXIS // normal axis
+    #define NORMAL_AXIS Y_AXIS
   #elif ENABLED(COREYZ)
     #define CORE_AXIS_1 B_AXIS // YZ from B + C
     #define CORE_AXIS_2 C_AXIS
-    #define CORE_AXIS_3 X_AXIS // normal axis
+    #define NORMAL_AXIS X_AXIS
   #endif
 
   /**

Marlin/temperature.h

                 babystepsTodo[CORE_AXIS_1] += distance * 2;
                 babystepsTodo[CORE_AXIS_2] -= distance * 2;
                 break;
-              case CORE_AXIS_3: // Z on CoreXY, Y on CoreXZ, X on CoreYZ
-                babystepsTodo[CORE_AXIS_3] += distance;
+              case NORMAL_AXIS: // Z on CoreXY, Y on CoreXZ, X on CoreYZ
+                babystepsTodo[NORMAL_AXIS] += distance;
                 break;
             }
           #elif ENABLED(COREXZ) || ENABLED(COREYZ)