Who is Speaking?

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 behind the entrance of the tent, and he (that men) was on the other side.

Sarah starts to laugh which is heard by the men outside.

Then in verse 13 it says that G-d replies to this by saying: “Why did Sarah laugh?”, “Is anything too difficult for God? 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 which speaks from the beginning or through his messenger? Because these messengers seems 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 (which were malachim) returning right? So how could one of them says: I will return?

How do I need to explain these things?

Rashi and the Radhbam seem to explain that one of the angels is speaking in 18:10 while G-d speaks in 18:14, but here’s the thing. Why would G-d speak through one of the messengers/angels in one case, but personally in another, while speaking about the exact same matter? If these angels are speaking and acting through the agency of G-d (like a shaliach), why couldn’t they speak out those words on behalf of G-d the second time as well? I’m not doubting what Rashi and the Rashbam say about these verses, I’m just trying to figure out why this would be the case (why G-d Himself speaks directly the second time around in opposite to the first time this message is delivered).