Responsiveness Spalten-Block anpassen - CSS-Lösung für das hier beschriebene Problem: https://die-netzialisten.de/responsiveness-spalten-block-anpassen/

style.css

/* 
Sources:
https://stackoverflow.com/questions/8720931/can-css-detect-the-number-of-children-an-element-has 
https://lea.verou.me/2011/01/styling-children-based-on-their-number-with-css3/
*/
@media (min-width:600px) and (max-width:781px) {

	.wp-block-column:first-child:nth-last-child(3) {
	  flex-basis: 100% !important;
	}

	.wp-block-column:first-child:nth-last-child(3) ~ .wp-block-column {
	  margin-left: 0 !important;
	}

}