Issue #3389101: Three column layout - widths as 33.33%

Closes #3389101

core/modules/layout_builder/layouts/threecol_section/threecol_section.css

@@ -26,11 +26,7 @@
   .layout--threecol-section--50-25-25 > .layout__region--first {
     flex: 0 1 50%;
   }
-  .layout--threecol-section--33-34-33 > .layout__region--first,
-  .layout--threecol-section--33-34-33 > .layout__region--third {
-    flex: 0 1 33%;
-  }
-  .layout--threecol-section--33-34-33 > .layout__region--second {
-    flex: 0 1 34%;
+  .layout--threecol-section--33-34-33 > .layout__region {
+    flex: 0 1 calc(100% / 3);
   }
 }