postsbycategory.php

<?php 

$ar = [];
$args = array(
	    'orderby' => 'id',
); 
$terms = get_terms( 'category', $args );

foreach ( $terms as $term ) :
	$ar[$term->name] = $term->term_id;
endforeach;

echo '<div class="posts-by-cat">';

foreach( $ar as $key => $value ) :

	$cache_key = 'posts_by_cat_' . $value;
	$cache_group = 'posts_by_cat';
	$posts_by_cat_query = wp_cache_get( $cache_key, $cache_group );

	if ( false === $posts_by_cat_query ) {
		$queryargs = array(
			'category__in'           => $value,
			'posts_per_page'         => 3,
			'order'	                 => 'ASC',
			'orderby'                => 'date',
			'no_found_rows'          => true,
			'update_post_meta_cache' => false
		);

		$posts_by_cat_query = new WP_Query( $queryargs );
		wp_cache_set( $cache_key, $posts_by_cat_query, $cache_group, 10 * MINUTE_IN_SECONDS ); 
	}

	if ( $posts_by_cat_query->have_posts() ) {
		echo '<ul class="' . $posts_by_cat_query->query_vars['category_name'] . '-posts">';
		echo '<h4 class="cat-posts-heading">' 
			 . '<a href="'. esc_url( get_category_link( $posts_by_cat_query->query_vars['cat'] ) )
			 .'" title="'. $key  .'">'
			 . $key
			 . '</a>'
			 . '</h4>';
		while ( $posts_by_cat_query->have_posts() ) {
			$posts_by_cat_query->the_post();
			echo '<li class="cat-post">' 
				 . '<span>' . get_the_title() . '</span>'
				 . '<button class="secondary-link read-more"' 
				 . 'data-href="' . esc_url( get_the_permalink() )
				 . '" data-title="' . get_the_title() . '">'
				 . '<i class="fa fa-angle-double-right"></i>Read More'
				 . '</button>'
				 . '</li>';
		}
		echo '</ul>';
	} else {
		// no posts found
	}
	wp_reset_postdata();

endforeach;

echo '</div>';