Who Were the Three Visitors to Abraham?
The Torah writes (Genesis 18:1-2):And the LORD appeared unto him by the terebinths of Mamre, as he sat in the tent door in the heat of the day; and he lifted up his eyes and looked, and, lo, three men stood over against him; and when he saw them, he ran to meet them from the tent door, and bowed down to the earth,
One to inform Sarah, and one to turn over Sodom, and one to heal Abraham for one angel cannot do two missions
"and behold three" - in gematria of "these were Michael, Gabriel and Rephoel"Yalkut Reuveni (ibid) cites another opinion as to their names:
Rephoel, Uriel and GabrielHowever, the Ralbag (ibid) learns that they were not angels but human beings:
He already raised his eyes before and saw three men and they were prophets ... and they are called angels in regards to Lot for a prophet is called an angel like we explained regarding the story of Hagar the Egyptian. But in regards to Abraham they were called men for they were not sent to Abraham for he himself was a prophet
And other commentators explain that these were three real men - who were prophets. If so, wasn't Abraham a prophet himself and why does a prophet come to another prophet unless he is greater like Moses and Aaron? And the answer is that they did not come to Abraham but for Sarah.
[see also a possible third opinion cited by the Ibn Ezra (ibid) and Sefer Mehokei Yehuda (ibid) explaining it]
When Did Milcah Have Children?
The Torah writes (Genesis 22:20):And it came to pass after these things, that it was told Abraham, saying: ‘Behold, Milcah, she also hath borne children unto thy brother Nahor
And from the language of the Torah it seems that Abraham did not about them until today, and if she had children in her younger years it would be impossible that he did not hear about them until today, for the distance between Aram Naharim and Eretz Canaan is not great. Also, Abraham was 75 when he left and also Nahor, his brother, was old and his wife was not young. But, G-d made a miracle for them by giving her children in her old age, and that is the reason for "Milcah also", and our Sages learn that she was remembered like her sister [Sarah]
Sforno (ibid) is not bothered by this question, and learns that she had children before:
We know already that Milkah bore children; now the point is to tell us that Betuel fathered Rivkah
When G-d told him "go from your land, your birthplace and the house of your father", included in that was a requirement to remove his mind from his father's house, and it was forbidden for him to mention them. But now that he elevated himself to a higher level and made it possible to attach himself to G-d, and there was no longer a possibility that he would return to his family, it was permitted for him to find out about them, and therefore [the news] were told to himThe Radak (ibid) learns that she had children before and now she had more:
It would appear that Milkah had stopped bearing children for a while already, but she had not been barren as had Sarah. Therefore Avraham was given the news that more children had been born for Nachor by his wife Milkah.