Here is an example of how to add the 'next' 3 events in the same category as the event linked to the current ticket into the cart. This will only work as expected with the Multi Event Registration add-on de-activated.

tw_ee_add_additional_regs_to_cart.php

<?php // Please do not include the opening PHP tag if you already have one

add_action('AHEE__EE_Ticket_Selector__process_ticket_selections__after_tickets_added_to_cart', 'tw_ee_add_additional_regs_to_cart', 10, 2);
function tw_ee_add_additional_regs_to_cart( $cart, $ProcessTicketSelector) {
    // Check we have an instance of EE_Cart.
    if( ! $cart instanceof EE_Cart ) {
        return;
    }
    // First, pull the 'first' ticket line item from the cart.
    $ticket_line_item = reset($cart->get_tickets());
    // Confirm we have an EE_Line_Item object before moving forward.
    if($ticket_line_item instanceof EE_Line_Item) {
        // Now pull the event linked to that ticket.
        $event = $ticket_line_item->ticket_event();
        // Confirm we have a EE_Event from above.
        if($event instanceof EE_Event) {
            // Get the first category assigned to the event.
            $category = $event->first_event_category();
            // Get the first datetime from the event.
            $datetime = $event->first_datetime();
            // Confirm we have both an EE_Term (category) and EE_Datetime from above.
            if(
                $category instanceof EE_Term
                && $datetime instanceof EE_Datetime
            ) {
                // Now pull another 3 events in the same category where the Datetime start date is greater than the current events datetime.
                $additional_events = EEM_Event::instance()->get_all(
                    array(
                        array(
                            'Term_Taxonomy.term_id' => $category->ID(),
                            'Datetime.DTT_EVT_start' => array('>', $datetime->start() )
                        ),
                        'order_by' => array('Datetime.DTT_EVT_start' => 'ASC'),
                        'limit' => 3
                    )
                );
                // Loop over each event and pull the active tickets from that event.
                foreach($additional_events as $additional_event) {
                    $tickets = $additional_event->tickets(            
                        array(
                            array(
                                'TKT_end_date' => array('>=', EEM_Ticket::instance()->current_time_for_query('TKT_end_date')),
                                'TKT_deleted'  => false,
                            ),
                            'limit' => 1
                        )
                    );
                    // Use the 'first' we foudn for the event.
                    $ticket = reset($tickets);
                    // Check we have a valid EE_Ticket before using it.
                    if( $ticket instanceof EE_Ticket ) {
                        // Add the ticket to the current cart.
                        $cart->add_ticket_to_cart($ticket);
                    }
                }
            }
        }
    }
}