We first meet the hyenas in The Lion King, when Simba & Nala arrives at the Elephant graveyard after scar have tricked Simba into going there. They ambush the two lion cubs shortly after they enter the graveyard, but the cubs manages to run away.
They follow the cubs, and after a while, manages to corner them. Mufasa arrives at the last moment, and stops them from hurting the cubs.

The three hyenas is responsible for the stampeede that kills Mufasa. Scar then tells them to kill Simba too, but he manages to outrun them, as they don't want to chase him through a barricade of thornbushes, beliving he would be dead out in the dry desert on the other side anyway.