Create a filter to randomize posts in a query

functions.php

// RANDOMIZE A SPECIFIC NUMBER OF POSTS
add_filter( 'the_posts', function( $posts, \WP_Query $query )
{
    if( $random = $query->get( 'jess_random_posts' ) )
    {
        shuffle( $posts );
        $posts = array_slice( $posts, 0, (int) $random );
    }
    return $posts;
}, 10, 2 );

// add this var to your query to use: jess_random_posts' => 3