Fix G29 with 3 point

Marlin/Marlin_main.cpp

       ABL_VAR int left_probe_bed_position, right_probe_bed_position, front_probe_bed_position, back_probe_bed_position;
       ABL_VAR float xGridSpacing, yGridSpacing;
 
-      #if ABL_PLANAR
+      #if ENABLED(AUTO_BED_LEVELING_LINEAR)
         ABL_VAR uint8_t abl_grid_points_x = GRID_MAX_POINTS_X,
                         abl_grid_points_y = GRID_MAX_POINTS_Y;
         ABL_VAR bool do_topography_map;
-      #else // 3-point
+      #else // Bilinear
         uint8_t constexpr abl_grid_points_x = GRID_MAX_POINTS_X,
                           abl_grid_points_y = GRID_MAX_POINTS_Y;
       #endif
 
       #if ENABLED(AUTO_BED_LEVELING_LINEAR) || ENABLED(PROBE_MANUALLY)
-        #if ABL_PLANAR
+        #if ENABLED(AUTO_BED_LEVELING_LINEAR)
           ABL_VAR int abl2;
-        #else // 3-point
+        #else // Bilinear
           int constexpr abl2 = GRID_MAX_POINTS;
         #endif
       #endif
 
     #elif ENABLED(AUTO_BED_LEVELING_3POINT)
 
+      int constexpr abl2 = 3;
+
       // Probe at 3 arbitrary points
       ABL_VAR vector_3 points[3] = {
         vector_3(ABL_PROBE_PT_1_X, ABL_PROBE_PT_1_Y, 0),
 
         #elif ENABLED(AUTO_BED_LEVELING_3POINT)
 
-          points[i].z = measured_z;
+          points[abl_probe_index].z = measured_z;
 
         #endif
       }