Create a simple Genesis grid loop by filtering the post class.

simple-genesis-grid.php

<?php

add_filter( 'post_class', 'prefix_simple_grid_class' );
/**
 * Add post classes for a simple grid loop.
 *
 * @category   Grid Loop
 * @version    1.0.0
 */
function prefix_simple_grid_class( $classes ) {
	global $wp_query;

	$current_post = $wp_query->current_post;

	$classes[] = 'simple-grid one-third';

	if ( 0 === $current_post || 0 === $current_post % 3 ) {
		$classes[] = 'first';
	}

	return $classes;
}