They are Raamaayan characters. They were Raavan's brothers. We meet them in Aranya Kaand after the incident of Shoorpankhaa's coming to the hut of Raam and Lakshman's cutting of her nose and ears. She goes to Khar and tells the whole incident. Khar gets very angry and asks that "You yourself are very powerful, then how did this happen to you? Who did this to you?" Shoorpankhaa told everything to him.

First he sent his 14 Raakshas to kill the two princes but they  themselves were killed. Then he sent 14,000 Raakshas army which was also killed. Then these three brothers came to battlefield themselves. After a while they also got killed.

Thus Raam alone killed Khar, Dooshan and Trishiraa with their 14 special Raakshas and 14,000 army.

Khar was like Vritraasur and Balaasur Raakshas who were killed by Indra's Vajra. He was like Namuchi who was killed by Indra by froth and foam. When Raavan heard about the killing of Khar and Dooshan, he thought that those people must be very brave and mighty who killed Khar and Dooshan.

Khar was a cursed being by Shiv Jee and he would be released from his curse by being killed by Raam, the son of Dasharath. In the Shesh Dharm of Mahaabhaarat, in the discourse between Bheeshm Pitaamah and Yudhishthir, it is said that sage Yaagyavalkya Jee had three sons - Chandrakaant, Mahaamedhaa and Vijaya. These three Braahman became Khar, Dooshan, and Trishiraa Raakshas by the Shaap of Shiv Jee. 

The first one Chandrakaant is Khar. And the disciples of the three Braahmaqn scholars were fourteen and they also became Brahm Raakshas. They were sent to fight with Raam in the beginning. On listening to Shoorpankhaa, Khar came to know that Vishnu has incarnated in the form of Raam and thus he wanted to get released from his Shaap by being killed by Raam, and hence firstly he sends his 14 Raakshas, who were their 14 disciples, and later these three brothers also follow them and thus get released from their Shaap.