Avraham sees three malachim. And two of those malachim appear to Lot in Sodom. What is the nature of those malachim? Are they malachim mamash, that is angels? Or are they malachim in the sense of messengers, in this case messengers from God, that is prophets.
Ralbag elsewhere makes various malachim into Neviim -- e.g. the one that appears to Manoach and his wife, and I saw that he did so as well in Vayera, talking about how the destruction of Sodom and Gemorrah happened at the hands of his neviim. So I thought I would look it up in Ralbag and lay out where he explains this in full. He does so in the very beginning of his commentary on Vayera.
He notes that Avraham lifts up his eyes and sees "anashim," men.
Now, we can say, as Speiser does in Anchor Bible, that of course these were really angels, but Avraham does not know this yet, and so we are not supposed to know this yet. It should be a surprise for us as well, with a gradual revelation in the story. And once the secret has been revealed, we can refer to them as angels in the story of Lot.
But that is not how Ralbag takes it. Rather, they are actually men. They are prophets, and thus messengers of God. (He suggests Shem and Ever as possible identities of these prophets.)
Saying that they are people, human beings, helps the plausibility of the narrative in several places. For example, there is the difficulty that angels to not eat, such that the midrash needs to explain that they only made it appear that they were eating, in order to give honor to Avraham. And also, a "malach" greets Hagar at some point, and she is not shocked. Ralbag says here that that malach was a human prophet. Otherwise, we are forced towards the answer provided by the midrash, that we see angels often frequent Avraham's house, such that this angel did not faze her at all.
We also see that Rivkah went to consult God. If there were indeed prophets of God running all about in those days, it makes it easier to understand how she would go to consult God on this score.
So why the difference between Lot and Avraham in terms of referring to them as malachim or anashim? Ralbag suggests that since Avraham often had prophecy, compared to him they were mere men. But to Lot, who was not accustomed to prophecy, they were malachim, prophets.
This is a great echoing and adapting of Rashi, echoing a midrash, on the sudden shift to calling them angels:
the…angels But elsewhere (18:2) Scripture calls them men! When the Shechinah was with them, it calls them men. Another explanation: In connection with Abraham, whose power was great, and the angels were as frequently with him as men, it calls them men, but in connection with Lot, it calls them angels. — [from Gen. Rabbah 52; Tan. Buber, Vayera 20]We would need, perhaps to explain the shift to men in the narrative with Lot. But that is clear from the fact that the men of Sodom are talking about these humans in their own speech, and so the shift is accomplished. And so the men -- and indeed they are men -- take hold of his hand and the hands of his wife and two daughters, and bring them out of Sodom.
There is also the fact that God speaks to Avraham in the preceding narrative. How so? Ralbag suggests this was a Divine revelation directly to Avraham, in the middle of the conversation, or else was a revelation to one of the prophets.
But did the malachim not perform open miracles in Sodom? Thus, in Bereishit 19:
might read this that they pulled him through a closed door. But in fact, we should read it as (e.g.) that they quickly opened the door a crack, grabbed Lot, yanked him indoors, and then closed the door behind them.
But they also smote them with blindness! Surely only an angel could do that?
Not so. We have precedent of a prophet doing this, namely Elisha, in II Kings 6:18:
So a prophet can do this, and with the same malady, sanverim.
They also overturned Sodom and Amora! Well, that was with God's actions, as the psukim themselves make clear. Or else, God empowered them to bring about such actions, just as God empowered Moshe in Egypt. So they can surely be human.
This might also go a long way to explaining the lack of power these angels seem to have. Lot brings them into the house because he is fearful for them, and he shuts the door behind the crowd. They let him initially offer his virgin daughters to the crowd, which the crowd rejects. One can certainly read this as them being in physical danger, and see that theme of palpable danger in the text, even though they were able to mount a defense of the house by smiting the people by the door with blindness.
So too the need to flee, and not delay to look back. They could not, apparently, provide a force field, as angels e.g. protected Chananiah, Mishael and Azarya, or Daniel, (or Avraham) in the furnace.
In terms of the dropping out of one of the anashim, he explains it similarly to the midrash, that one of them was only commanded about informing Sarah, and so his particular mission was over. This has to do with the limitation of these prophets' prophecy.
Despite all this, I am not entirely convinced that that is the meaning of malachim here. It makes it more palatable for rationalists who don't like the idea of angels walking around doing things. And perhaps the likes of Rambam would not need to see it all as an inspired day-dream. But on the other hand, who says that we must read our own biases into the Biblical text? And indeed, if one wants to read the meeting of the malachim with Avraham as one in which one of the "anashim" was Hashem himself (see other posts on this topic here), it would make more sense for Hashem to be accompanied by angels rather than prophets.