Horford was hit with a flagrant-2 foul for elbowing Cavaliers guard Matthew Dellavedova after they got tangled up in the final minute of the second quarter.
As a result of Horford's ejection, the Hawks lost the game 114-111 in overtime, which gave the Cavaliers a 3-0 series lead.
"I'll definitely learn from this and will make sure to keep my composure and things like this don't happen," Horford told reporters. "But the game before I got hit in the knees and it kind of played over again."
The Cavaliers will book their spot in the NBA finals if they manage to beat the Hawks in Game 4 on Tuesday.