Section 3 Summary

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

Nothing lasts.
There is a graveyard where everything I am talking about is,
now.

Yup, that's true. It's also kind of a bummer.
In the beginning of section 3, the speaker has yanked us out of that lovely barn and pulled us into the present, where the barn, and that family of hers are no longer.
In the place of the bar, we have a graveyard, where everything she was talking about is. That includes the barn, the family, all those patient animals.
But hang on a second. She still remembers these things, right? They may be buried, but she's still describing them on paper.
Bingo. This is all about memory—the way we hold on to little bits of the moments, things and people that are no longer with us.
The memory of those things, at least, does last. But we guess even that will go, once all the people who remember those things are, themselves, gone.

I stood there once, on the green grass, scattering flowers.

This graveyard? The speaker totally mourned at it—even going so far as to scatter flowers on the graves of the things that are no longer there.
But here's the thing. She's not literally talking about a graveyard, right? It's gotta be a metaphor, but for what?
We think our speaker is almost saying something like this: I spent some time visiting the memories of the things I've lost. I mourned their passing.
But since she's not there anymore (she "stood there once"), that seems to suggest that she's moved on.