Conditional redirect to several pages

So you need this to happen before Wordpress has output anything at all, otherwise you won't be able to send a redirect.

A way to achieve this is with the template_redirect hook, documented here: https://codex.wordpress.org/Plugin_API/Action_Reference/template_redirect

The example from that docs page is more or less exactly what you want. I've edited it a little bit for your question:

function my_page_template_redirect() {
    if ( is_page( 'page-main' ) ) {
        if ( $some_condition ) {
            wp_redirect( home_url( $pageA ) );
        } else { 
           ... other caeses here with redirects to other pages
        }
    }
}

add_action( 'template_redirect', 'my_page_template_redirect' );

You'd need to add that to your functions.php or a plugin.