functions.php

<?php

/**
 * Insert CTA after first row of grid 
 * @author Bill Erickson
 * @see https://www.billerickson.net/a-better-and-easier-grid-loop/#comment-699401
 */
function be_cta_after_first_row() {

  // Only run on the grid 
  $grid = apply_filters( 'is_genesis_grid_loop', false );
  if( ! $grid )
    return;
    
  // Only run before the 4th post (3 column grid )
  global $wp_query;
  if( 3 != $wp_query->current_post )
    return;
    
  // Display CTA
  echo '<div class="cta-box"><h4>My CTA</h4><p>Lorem ipsum dolor sit amet</p></div>';
}
add_action( 'genesis_before_entry', 'be_cta_after_first_row' );