Stanza 3 Summary

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

Now they are flying in place, conveying
The terrible speed of their omnipresence, moving

Let's break these lines down.
"Conveying" in this case means showing or communicating. "Omnipresence" means being everywhere at once.
Describing their speed as "terrible" might mean that it's something to be in awe of, something really impressive.
Together, the lines mean something along the lines of, while they're hovering over the world, the angels have the potential to zoom anywhere at any time. They are angels, after all.
Or wait. We thought they were bed-sheets. Can they be both? Metaphorically speaking, absolutely.
Maybe the laundry is a metaphor for angels. Maybe the angels are a metaphor for laundry. Either way, it certainly seems like the laundry is being figuratively described as angels.
So whoever this speaker is, he's a guy who can see beautiful, spiritual images in something as mundane and every day as drying laundry. He's either got awesome vision or an excellent imagination.
Whatever the case, the speaker's not done with this idea yet. He's enjambed line 13 with line 12, so we'll have to keep reading to understand his finished thought.

And staying like white water; and now of a sudden
They swoon down into so rapt a quiet
That nobody seems to be there.

Those flying angels? They're moving and staying. Weird right? How can something move and stay?
Luckily, the speaker answers this question for us by comparing these laundry angels to white water in a simile in line 14.
The idea is that, though the angels are stuck pinned to the laundry line, they're still full of motion, probably thanks to that fresh breeze we've got going on.
But then, things change. All of a sudden they "swoon" or swoop down and become totally still in "rapt quiet."
Wilbur's working in another pun here. "Rapt" means to be captivated by something, but it also means to be carried away to heaven, as in the Rapture. These are angels, we're talking about, so the second definition fits just as well as the first.
After they descend, they become so quiet that it seems as if no one is there at all. Perhaps they've gone back to heaven? This might be a sign that it's time for humans to wake up. Away with the souls, the angels, and all the otherworldly stuff.

The soul shrinks

The soul shrinks? What, does it get tinier or something?
Probably not. We're thinking that "shrinks" here means that the soul shies away from something, or cowers. Whatever's coming, it's not a fan.
In any case, the soul is sneaking into the background here, slinking away as the scene changes. Prepare yourselves for the body to enter the fray.