Hi Mary -
The b-ticket website states:
https://webshop.b-ticket.com/webshop/webticket/timeslot
"The 72-Hour Cumulative Pass allows admission to the Galleria dell’Accademia and the Bargello Museums (Museo del Bargello, Medici Chapels, Palazzo Davanzati, and Orsanmichele) under a single ticket. Visitors may enter the Bargello Museums on the same day as their visit to the Galleria dell’Accademia and up to the following 72 hours. The date and time slot selected at the time of purchase correspond to the entrance time for the visit to the Galleria dell’Accademia."
So you only have to pre-reserve an entry time for the Accademia at the time you buy the pass, and can visit the others on just a walk-up basis within 72 hours after that entry: no timed-entry reservation needed for them. Make sense?
Given the individual pricing (listed in this page from The Florentine and which reportedly have seen increases this year) this is a good buy: 38 euro for what would otherwise total 61 euro if bought separately, not including ticket reservation fees. As you have 3 days, I'd really try to see the Orsanmichele Complex and Palazzo Davanzati Museum as well!