The third season of the Australian reality television series, Big Brother, also known as Big Brother 2003, lasted 86 days, starting on 27 April 2003 and ending on 21 July 2003. The winner of season 3 was Regina Bird. Regina earned public admiration with her down-to-earth charm and work ethic. Chrissie Swan was the runner-up for the season and has excelled as an entertainment and media personality since. Bird was the first female winner of the show. Peter Abbott, the show’s executive producer, also served as the voice of Big Brother, with this season being the last in which he performed the role.

The season follows the same basic format as the prior season, however the twist for the season had housemates starting their Big Brother experience in two separate houses - a "Round House" and a "Square House". On Night 22, all Housemates were locked into their respective bedrooms and a construction crew spent the night merging the two houses and revealing the hidden swimming pool and kitchen that had been in between the two smaller houses. Housemates were released into their new combined house on the morning of Day 23. Later in the day, a bathroom linking the two bedrooms was revealed. This was also the first season in which cameras were completely hidden for the first time in this season. Previously some cameras were visible to housemates and at times housemates were aware of the camera tracking their movements. The season was the last to permit housemates to read books in the house, reflecting the more lenient rules of the early seasons before stricter restrictions were introduced from 2004 onwards.

Two new shows were introduced:

The Tower of Terror is displayed, and the logos of Dreamworld, Big Brother Australia, Network Ten and a map of Australia turns into a housemate-selection machine. We go through the orange bedroom, through the blue bathroom for a shower, through to the blue control room, to the edit section where footage from the diary room is being edited to be shown on the show before progressing to the garden (or the Big Brother Australia yellow stage for Nominations and Evictions). We then go to the purple kitchen, and three housemates are up for nomination the red housemate gets the most votes. The pink dining room is broadcast to a person on the internet at home. After 85 days, a winner is selected.

Fourteen Housemates entered during the first week of the series and were sent to one of two houses - the Square house (which was on the right of the compound) originally featured Belinda, Jaime, Irena, Carlo and Claire with Vincent and Saxon joining later in the week. After Ben entered the Round house on Day 1, Joanne, Regina, Chrissie, Daniel, Patrick and Leah were progressively admitted over the first week.

Regina Bird returned to compete again on Big Brother 2022, and won, making Regina the first player of Big Brother Australia to win twice.

In February 2015, Chrissie Swan appeared on the first season of I'm a Celebrity Get Me Out of Here!, finishing in third place. She also appeared on Celebrity MasterChef Australia, being first to be eliminated.

Regina Bird appeared on the 11th season of I'm a Celebrity Get Me Out of Here!, and finished as runner-up.