The exciting 24-year-old, who was recruited from the Oakleigh Chargers with pick 48 in the 2006 National Draft, inked the new deal last week and Crows football operations manager Phil Harper said Mackay was always keen to-resign.
"There was never any issue with David's contract, it was just about getting the timing right for him," Harper said.
"It still is very pleasing to get it done before the finals so he can focus on that.
"He is an outstanding young man and very talented footballer who is an extremely valuable member of our team."
Mackay has played 83 AFL games since making his debut in the opening round of the 2008 season, and has played 20 of the Crows' 22 games this season.
"It's really exciting to get it all done," Mackay said. "I'm really looking forward to being a part of the Club for the next three years and it starts this week."I can't wait to get out there on the weekend."