Get comments only for certain specific users in template file

You can try to modify the query using the comments_clauses filter:

$args = array( 'user_id' => 0 );
add_filter( 'comments_clauses', 'custom_comments_clauses');
$comments = get_comments( $args );
remove_filter( 'comments_clauses', 'custom_comments_clauses');

function custom_comments_clauses( $clauses ){
    $clauses['where'] = str_replace( 'user_id = 0', 'user_id IN (1, 2, 3)', $clauses['where'] );
    return $clauses;
}

Update:

You could also add a custom parameter, my_user_ids, to the query like this:

$args = array( 'user_id' => 0, 'my_user_ids' => array(1,2,3) );

add_filter( 'comments_clauses', 'custom_comments_clauses', 99, 2 );
$comments = get_comments( $args );
remove_filter( 'comments_clauses', 'custom_comments_clauses', 99, 2 );

function custom_comments_clauses( $clauses, $query ){
    if( isset( $query->query_vars['user_id'] ) && isset( $query->query_vars['my_user_ids'] ) ){ 
        $user_id = $query->query_vars['user_id'];
        $user_ids = $query->query_vars['my_user_ids'];
        if( count( $user_ids ) > 0 ){
            $from = sprintf( 'user_id = %d', $user_id );
            $to   = sprintf( 'user_id IN (%s)', join( ',', $user_ids ) );
            $clauses['where'] = str_replace( $from, $to, $clauses['where'] );
        }
    }
    return $clauses;
}

where $my_user_ids is an array of user ID’s.