Stanza 3 Summary

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

I will arise and go now, for always night and day
I hear lake water lapping with low sounds by the shore;

Just in case we weren't sure, the speaker is all: Guys? I'm really going. I'm going to Innisfree. And don't you forget it.
In a way, it's almost as if he snaps out of his daydream with fresh resolve and determination. That daydream he was describing—the plans he has to build his cabin—was so real it was almost as if he were already there.
But he's not. Still, wherever he is, he can hear the lapping of the water on Innisfree in his mind or something.
Of course because he's not really hearing the water, we should think of this auditory image as a metaphor for his strong desire to head to Innisfree, ASAP.
We think it's safe to say that he imagines the sounds because he's so absorbed in his desire to get to this ideal place.
And "for always night and day" lets us know that he's pretty haunted by the place—obsessed even. No wonder he's so determined.

While I stand on the roadway, or on the pavements grey,

Ah, now we get it. Here's hard evidence that the speaker is not in a rural place, but one with paved roads. In other words, he couldn't be further away from idyllic, rural Innisfree.
The fact that he can hear the waters of Innisfree while he's standing on cold, hard pavement is a testament to just how much he loves that place.
Plus, check out that sneaky little internal rhyme here. Roadway rhymes with grey, but they don't both come at the end of the line (hence calling it "internal rhyme"). It's the kind of thing you don't notice unless you're reading the poem aloud.
So do it, Shmoopers. Read this guy aloud. You won't regret it. We promise.

I hear it in the deep heart's core.

Bingo. It's confirmed. The lapping water the speaker hears is imaginary.
While we suspected that Innisfree is, at this point, mostly a figment of this dude's imagination, his reference to the "deep heart's core" shows us just how important Innisfree is to him.
It's not just something he's thought about once or twice, but something that he desires deep down inside. He doesn't just hear the water in his mind; he hears it in his heart. There's an emotional connection there, don't you think?
And of course the heart can't actually hear, so we might think of the "deep heart's core" as a metaphor for the part of the speaker that feels and dreams deeply.
The end of this poem is pretty sad. Think of the movement of the poem. The speaker sets up this ideal place where he feels completely at peace. We kind of get that he's fantasizing, but we don't realize how sad it is until the ending, when we find out he's standing in a place so completely different from his dream.
Instead of getting the sense that he's close to reaching Innisfree, the poem ends with a picture of him standing on a paved road pining over a place he might never get to.
Bummer, dude.