What was Daenerys' third sacrifice for birth of her dragons?

Question

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}

and

“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.}

Answer 1

We don't know

As discussed in the comments & original question, what we do know is that Mirri told Dany that "only death may pay for life". What we don't know is if she's correct, nor what is the exchange rate on deaths, nor do we know the shelf life of the exchange (i.e. when does the death have to occur to fuel the life).

Unreliable Reporting

I can't find the quote at present, but in times past GRRM has said his characters are often unreliable narrators. So, to start with we don't know factually that Mirri's statement "only death can pay for life" is true at all.

For that matter, we don't even know that sacrifice was needed at all. The fact that other Targaryens tried hatching dragons and failed while Dany succeeded could have been linked to the rise of magic (demonstrated by the comet; it's never clear if the dragons bring the magic or the magic brings the dragons) or perhaps Dany herself has some attribute the others did not and so the fires alone would have been enough (this is hinted at with her affinity for the eggs and sense something's inside, much like the Stark children's warging).

Life force exchange rate

Even if her words are 100% accurate, there's wiggle room. The statement does not imply there's a 1 life to 1 death cost, nor do we know the exchange rate on deaths to lives. Does a dragon's life cost the same as a human's? Does the current power or age of the life matter? Without a means to receive change for partial lives spent, we cannot know that the cost of Rhaego to resurrect Drogo was a fair exchange. Perhaps the little soul was only 27% expended in the effort, but the other 73% was lost for lack of use. We don't know.

Freshness label

Again, we don't know proximity of the death:life. The horse was killed before the fire started, but so was Drogo. Others lost their lives after. We have not heard anything yet indicating which of those lives might have influenced the blood magic, but one could argue that all of the possibilities were needed, not just 3, if the exchange rate (see above) was such that dragon lives are more expensive than human. Conversely, the eggs themselves may have already had a tiny spark of life in them (hence Dany's feelings of warmth and affinity from them), which may have meant they were more 'fresh' than a recently killed corpse and therefore needed less of a jolt from the sacrifice, if one was even needed.

Answer 2

It was Drogo, Rhaego and Miri Maaz Dur.

“They are mine,” she said fiercely. They had been born from her faith and her need, given life by the deaths of her husband and unborn son and the maegi Mirri Maz Duur. Dany had walked into the flames as they came forth, and they had drunk milk from her swollen breasts. “No man will take them from me while I live.”
_{ACOK - Daenerys I}