Hello Sir!
In Bucharest, only pensioners with a permanent residence in the city benefit from free of charge, regardless of the amount of the pension, based on the travel card for pensioners or on the pension coupon and the ID. (See information here: https://www.stbsa.ro/eng/reduceri-si-gratuitati_eng)
If you stay only 3 days in Bucharest, we recommend you to use a 72-hour pass (20 lei=around Eur 4). In order to purchase it you can use Mobile ticketing app 24Pay or you can buy a card from a kiosk or from a vending machine (we have one vending machine right at the Otopeni International Airport). Positioning of kiosks and vending machines here: https://www.google.com/maps/d/edit?mid=1nw1ZJHzIO_2JA3d5PnySq8lxKSb2g9hH&usp=sharing.
In some buses and other type of vehicles it is possible to pay by contactless bank card directly to the validating machine in the vehicle.
If you need to know more about travelling in Bucharest, you can use this link: https://www.stbsa.ro/index.
Best regards,