One work-around without having to customize code is to first enter one beer, apply discount, and then add the two last beers.

I see the possible headache when the customer gives you the voucher AFTER you've already added all the beers though..

If you know how to do basic odoo development, removing the if statement from line 1985 in the models.js file (point_of_sale module), and make it always run the "else" clause (in bold), it will stop merging lines. 
This should of course be done by creating a custom module and overriding the the add_product function.

if( last_orderline && last_orderline.can_be_merged_with(line) && options.merge !== false){ 

            last_orderline.merge(line);

        }else{

            this.orderlines.add(line);

        }