Genesis 18 who is the ‘he’

Question

Avraham stands at the entrance of his tent after his Brit - Bereshit 18 - looking around he sees three men at a short distance (even getting closer).

At a certain point they ask Avraham: “Where is your wife Sarah?” Avraham replies: “See (here) in the tent.”

Then one of the men (he) says: “I will return to you and your wife Sarah will have a son.” Sarah was listening from the tent and as a reaction she starts to laugh.

Then in verse 13 it says that G-d replies to this by saying: “Why did Sarah laugh?”, “Is anything too difficult for G-d”? At the designated time, I will return, and Sarah will have a son”

So here is my question: is the he really one on the men? Or is it G-d who speaks from the beginning or through his messenger? Because these messengers seem to be close to the tent and hear Sarah laughing. And ‘he’ says: I will return, while later it’s clear G-d says He will return. And again in Genesis 21:1 this seems to be the case. We read nothing of one of those men (who were malachim) returning right? So how could one of them says: I will return?

How do I need to explain these things?

Answer 1

Rashi to Genesis 18:10 (the first verse you cite with "I will return to you") explains that the angel is speaking on behalf of G-d.

שוב אשוב. לֹא בִשְּׂרוֹ הַמַּלְאָךְ שֶׁיָּשׁוּב אֵלָיו, אֶלָּא בִשְׁלִיחוּתוֹ שֶׁל מָקוֹם אָמַר לוֹ, כְּמוֹ וַיֹּאמֶר לָהּ מַלְאַךְ ה' הַרְבָּה אַרְבֶּה, וְהוּא אֵין בְּיָדוֹ לְהַרְבּוֹת, אֶלָּא בִּשְׁלִיחוּתוֹ שֶׁל מָקוֹם, אַף כָּאן בִּשְׁלִיחוּתוֹ שֶׁל מָקוֹם אָמַר לוֹ כֵּן

"I will surely return." The angel did not tell him that he will return to him, but rather through the agency of G-d did he say this to him. This is like "The angel of G-d said to her, 'I will surely increase'" (Genesis 16:10). It is not in [the angel's] power to increase, but rather through the agency of G-d. So, too, here, through the agency of G-d did he said this to him.

In other words: The "he" is one of the angels, but he is speaking for G-d. (Let me emphasize this: G-d is not one of the men.) G-d says "I" the second time, speaking for Himself, and, as you point out, G-d is indeed the one who "returns" to grant Sarah a son in 21:1.

Answer 2

The Bartenura learns that it was the door that was behind the visitors, and they couldn't see Sara. Not that she was in the doorway. The stress is that she was extremely modest and would not have laughed out loud enough to be heard.

As opposed to your assertion that "Sarah starts to laugh which is heard by the men outside." (which is actually contrary to the explicit verse that she laughed to herself - וַתִּצְחַ֥ק שָׂרָ֖ה בְּקִרְבָּ֣הּ.)

והוא אחריו הפתח היה אחר המלאך. בא רש"י לפרש שלא תאמר דמה דכתיב והוא אחריו שהמלאך היה אחר הפתח כמשמעות דקרא דאם כן היה צריך לומר ששרה היתה פתח האהל מבחוץ וזה אינו שהרי כתוב הנה באהל צנועה היא:‏

So when you translate הנה באוהל as “See (here) in the tent.” it's incorrect. It should be "behold! In the tent", since the couldn't "see" her.

So the angles reported the news, nobody heard Sara's reaction and then Gcd gave a promise that He'd return a year later.