If a husband says to his wife: "If you speak to so-and-so again, it's by the three [divorces]," is this considered an ambiguous divorce or a conditional divorce, and what is its ruling, knowing that he intends it as a threat and a warning and did not utter the word "divorced"?

The phrase mentioned in the question is not an explicit divorce pronouncement; rather, it is a veiled statement (kinayah). Divorce does not occur through it unless the husband intended to enact it. Therefore, if the husband's intention with the phrase was to threaten and he did not intend to enact divorce, then no divorce occurs if the woman speaks to the person he forbade her from speaking to.