This is an interesting question because Calvin tries to explain the three characters to the beasts continually as Meg is recovering from her escape from the "Black Thing" in Chapter 11. However, it is only when Meg begins to try to explain them that Calvin has an epiphany:
“Angels!” Calvin shouted...
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suddenly from across the table. “Guardian Angels!” There was a moment’s silence, and he shouted again, his face tense with concentration, “Messengers! Messengers from God!”
The problem arises because the beasts of this planet cannot see. Instead, they can feel and experience a sort of mental telepathy. Both Meg and Calvin are trying to explain what the three women looked like to the beasts; however, they are unable to understand anything about seeing. One of the beasts tries to explain by saying that the children should “think about what they are.” Calvin, who is used to seeing things and describing things that way, finally realizes the closest thing to what the three women “are.” Calvin does a good job by describing them as “angels,” but it still is not enough for the beasts to understand the three women. It is at this point that the three women appear. It is only the appearance of Mrs. Whatsit, Mrs. Who, and Mrs. Which that finally helps the beasts understand.