Is the phrase "Consider yourself to have taken the third [divorce]," which the husband sent via text message in a moment of anger and intended as a threat, considered an irrevocable divorce (ṭalāq bā'in), given that it was preceded by only one divorce?

No divorce occurs by the husband's statement: "Consider yourself to have received the third [divorce]," as long as he did not intend divorce by it. This is because it is one of the ambiguous expressions (kinayat) that require intention for divorce to take effect.