The club had delayed its decision on a number of players until after the season but saw a role for each player as Melbourne re-shapes its list.
"All three were consistent in the second half of the season and played a role in steadying the ship," Melbourne list manager Tim Harrington said.
"They played a particular role at different times that was valuable for the team."
The Demons were keen to retain Dunn, an unrestricted free agent, after the 25-year-old performed well in defence during the second half of the season.
He eventually finished 10th in the club best and fairest and with 99 games behind him looks capable of cementing himself in the Demons senior team.
MacDonald played 16 games in 2012 and impressed with his professional attitude and solid approach.
Once Jared Rivers joined Geelong, he looked certain to be retained as the Demons recognised the value of his experience.
The 28-year-old MacDonald has now played 123 games, having joined Melbourne in 2010 after 80 matches with the Brisbane Lions.
Melbourne gave Sellar a lifeline when it selected him with pick No.54 in the 2011 NAB AFL Draft.
The 23-year-old tall had played 21 matches in four years with Adelaide after being a first round selection in the 2006 draft.
He adds depth and versatility to the Demons list and played 16 games for the club in 2012.
Both Sellar and Dunn were first-round draft picks while MacDonald emerged at the Lions as a rookie.
