Stanza 3 Summary

Get out the microscope, because we’re going through this poem line-by-line.

O love, they die in yon rich sky,
They faint on hill or field or river:

Hmm. This line is a bit ambiguous—who is the "love" the speaker is talking to? This is the first time the speaker addresses a specific person, but we're not sure who it is, exactly.
The speaker tells his "love" that "they die" in "yon" (short for "yonder," or "over there") sky.
But what is it that "dies"? It probably means the "echoes" of the bugle horns from the previous stanzas, but it's not totally clear what the word "they" refers to.
More ambiguity!
The "echoes," then, fade away across the "hill," "field," and "river" in the valley.
But the speaker chooses to use words like "die" and "faint," which are things that people would do. So again—is he talking about the echoes fading away and merely personifying them? Or is he thinking about actual people getting old and dying? Or is it some of both? What do you think?

Our echoes roll from soul to soul,
And grow for ever and for ever.

Now the speaker uses the first person plural to describe the "echoes" of the bugle. He calls them "our echoes."
But he wasn't the one blowing the bugle horn. Is he really still talking about the horn? Or is he thinking about the "echo" that a person might leave behind after he or she dies? Something more like a memory or a legacy?
The speaker clearly thinks that the valley is somehow magical or mythical, since he imagined that the bugle echoes might be coming from "Elfland." That magical, mythical beauty seems to have inspired him to get all deep and philosophical on us.
He says that those "echoes"—whether they are literal echoes from the bugle, or the figurative "echoes" of a person's life—move "from soul to soul." In other words, these "echoes" connect people on an almost spiritual level.
The "echoes" he's talking about now don't "die" like the bugle echoes do—these "echoes" keep growing "forever and forever."

Blow, bugle, blow, set the wild echoes flying,
And answer, echoes, answer, dying, dying, dying.

Again, the speaker asks the bugle to blow the notes again because he wants to hear the echoes.
The echoes answer back as they bounce off the mountains (just as these last lines "echo" the final two lines of stanzas 1 and 2) and then die away.