As with the second stanza, the third picks up right where the second left off (this pattern will continue throughout the poem). It gives it a nice figure, don't you think?
Apparently some mysterious "they" names the woman before the speaker. In other words, various other aristocrats and elites talk about the woman in front of him. They gossip, as it were.
Whenever this happens, it's like a "knell" to the speaker's ear. A knell? Yeah, a knell. You know, that word meaning the sound of a bell.
But it's not just any old bell. Usually, a "knell" refers to the somber, slow, sound of a bell being rung to announce a death, the so-called "death knell."
This word is never really a good word. Thomas Gray, for example, talks of how the "curfew tolls the knell of parting day."
The point is, even the mention of the lady's name in front of the speaker is painful. Remember, he and she have said their goodbyes, and he is none too happy about it.
The name itself sounds like a death knell for our speaker, which here is a metaphor for how even the uttering of the woman's name makes the speaker think of death—the ultimate departure or end.