What you want to do is filter the posts in the query itself using meta_query.
$args = array(
'post_type' => 'activities',
'meta_query' => array(
'key' => 'rw_activities_page',
'value'=> $postid,
'compare'=>'='
),
'orderby' => 'date',
'order' =>'ASC',
'posts_per_page' => 3);
You might need to adjust the meta_key if that's not exactly the value it's using.