The grueling contest took place with Miami's LeBron James and Dwyane Wade on the bench, both trying to recover from mild injuries. Wade has a sore foot and James suffered a slight ankle sprain in Miami's win Wednesday over Indiana.
Atlanta, which handed Miami its only loss of the season Monday, had a chance to take a four-point lead with 10 seconds to go in regulation, but Joe Johnson could make only 1-of-2 foul shots. That gave Chris Bosh the opportunity to tie the game with a 3-pointer in the final second.
Chalmers then hit a 3-pointer with 1 second left in the first extra period to again tie the contest and Chalmers scored the final two points of the second overtime from the foul line.
Having escaped on three occasions, the Heat shut out Atlanta in the third extra session. Chalmers scored the first of Miami's seven points in the 5-minute period with his 3-pointer and he finished with 29 points. Bosh scored 33.
Atlanta was led by 20 from Johnson, one of six Hawks to score in double figures.
Not only did the game stretch well past the 3-hour mark, it came during one of the typically difficult stretches teams will be facing during the lockout-shortened season.
Miami was playing its fourth game in five days. Atlanta will play Friday and Saturday as part of five games in six days.