Daenerys paid for her three dragons with three lives1. Two are obvious i.e. Khal Drogo and Mirri Maz Duur.
Inside the tent Dany found a cushion, soft silk stuffed with feathers. She clutched it to her breasts as she walked back out to Drogo, to her sun-and-stars. If I look back I am lost. It hurt even to walk, and she wanted to sleep, to sleep and not to dream. She knelt, kissed Drogo on the lips, and pressed the cushion down across his face.
AGOT - Daenerys IX
The sun was going down when she called them back to carry his body to the pyre. The Dothraki watched in silence as Jhogo and Aggo bore him from the tent.
AGOT - Daenerys X
“Is it so far from madness to wisdom?” Dany asked. “Ser Jorah, take this maegi and bind her to the pyre.”
“To the... my queen, no, hear me...”
“Do as I say.” Still he hesitated, until her anger flared. “You swore to obey me, whatever might come. Rakharo, help him.”
The godswife did not cry out as they dragged her to Khal Drogo’s pyre and staked her down amidst his treasures. Dany poured the oil over the woman’s head herself. “I thank you, Mirri Maz Duur,” she said, “for the lessons you have taught me.”
“You will not hear me scream,” Mirri responded as the oil dripped from her hair and soaked her clothing.
“I will,” Dany said, “but it is not your screams I want, only your life. I remember what you told me. Only death can pay for life.”
AGOT - Daenerys X
The third one is ambiguous.
There was a horse that was burnt with Drogo and Mirri.
Rakharo chose a stallion from the small herd that remained to them; he was not the equal of Khal Drogo’s red, but few horses were. In the center of the square, Aggo fed him a withered apple and dropped him in an instant with an axe blow between the eyes.
Bound hand and foot, Mirri Maz Duur watched from the dust with disquiet in her black eyes. “It is not enough to kill a horse,” she told Dany. “By itself, the blood is nothing. You do not have the words to make a spell, nor the wisdom to find them. Do you think bloodmagic is a game for children? You call me maegi as if it were a curse, but all it means is wise. You are a child, with a child’s ignorance. Whatever you mean to do, it will not work. Loose me from these bonds and I will help you"
When a horselord dies, his horse is slain with him, so he might ride proud into the night lands. The bodies are burned beneath the open sky, and the khal rises on his fiery steed to take his place among the stars. The more fiercely the man burned in life, the brighter his star will shine in the darkness.
AGOT - Daenerys X
So Mirri clearly thought that the horse was being sacrificed to make the blood magic work. Her sole objection was that Daenerys did not know the spells to make it work, not that the horse was not a suitable sacrifice. That has been my view and interpretation.
But there are people who disagree and suggest that it was in fact Rhaego. Rhaego was sacrificed to save Drogo's life so is it possible to use him twice as a sacrifice? Some claim that Rhaego's soul was inside Drogo so killing Drogo actually counts for two sacrifices and the third one is Mirri. There are also some people who claim that it may have been Daenerys herself, even though she did not actually burn and die, her willingness to walk into the pyre counts as a sacrifice.
What I am seeking is something that George or his associates (w.r.t Books, not show) may have said about that, settling it once and for all, not stirring another debate on the most likely candidate.
1. Given that only death can pay for life and one death is needed for granting someone life, It stands to reason logically that to bring three dead stone eggs to life, three deaths shall be needed.