Hi Erin -
On a whim, you should check the ticket that you purchased to make SURE it includes the Accademia? Other than the 72-hour Firenze Card, which includes much more than the 3 museums you listed, I'm unaware of any ticket which includes only the Uffizi, Accademia and Pitti. Maybe I've missed something?
If it was the Firenze Card that you purchased, there are instructions on the website for making timed-entry reservations for both the Uffizi and Accademia.
http://www.firenzecard.it/en/faq
Otherwise, make sure that what you purchased wasn't the "PassePartout 3 Days" combo ticket for the Uffizi, Pitti Palace, Boboli Gardens and Archeological Museum.
https://www.uffizi.it/en/tickets
https://www.uffizi.it/en/pages/combined-ticket
That particular ticket allows you to make a timed-entry reservation for the Uffizi but not for the others it includes as queues are never long enough at those to be of concern. It doesn't include the Accademia.